Display device

ABSTRACT

With an increase in the definition of a display device, the number of pixels is increased, and thus the numbers of gate lines and signal lines are increased. Due to the increase in the numbers of gate lines and signal lines, it is difficult to mount an IC chip having a driver circuit for driving the gate and signal lines by bonding or the like, which causes an increase in manufacturing costs. A pixel portion and a driver circuit for driving the pixel portion are formed over one substrate. At least a part of the driver circuit is formed using an inverted staggered thin film transistor in which an oxide semiconductor is used. The driver circuit as well as the pixel portion is provided over the same substrate, whereby manufacturing costs are reduced.

TECHNICAL FIELD

The present invention relates to a display device in which an oxidesemiconductor is used and a method for manufacturing the display device.

BACKGROUND ART

As typically seen in a liquid crystal display device, a thin filmtransistor formed over a flat plate such as a glass substrate ismanufactured using amorphous silicon or polycrystalline silicon. Thinfilm transistors manufactured using amorphous silicon has low fieldeffect mobility, but can be formed over a larger glass substrate. Incontrast, thin film transistors manufactured using crystalline siliconhas high field effect mobility, but due to a crystallization step suchas laser annealing, the transistors are not always suitable for beingformed over a larger glass substrate.

In view of the foregoing, attention has been drawn to a technique for bywhich a thin film transistor is manufactured using an oxidesemiconductor and such a transistor is applied to an electronicappliance or an optical device. For example, Patent Document 1 andPatent Document 2 disclose a technique by which a thin film transistoris manufactured using zinc oxide or an In—Ga—Zn—O-based oxidesemiconductor for an oxide semiconductor film and such a transistor isused as a switching element or the like of an image display device.

PATENT DOCUMENT

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-96055

DISCLOSURE OF THE INVENTION

The field effect mobility of a thin film transistor using an oxidesemiconductor for a channel formation region is higher than that of athin film transistor using amorphous silicon. The oxide semiconductorfilm can be formed by a sputtering method or the like at a temperaturelower than or equal to 300° C. Its manufacturing process is easier thanthat of a thin film transistor using polycrystalline silicon.

Such an oxide semiconductor is expected to be used for forming a thinfilm transistor on a glass substrate, a plastic substrate, or the like,and to be applied to a liquid crystal display device, anelectroluminescent display device, an electronic paper, or the like.

With an increase in the definition of a display device, the number ofpixels is increased, and thus the numbers of gate lines and signal linesare increased. Due to the increase in the numbers of gate lines andsignal lines, it is difficult to mount an IC chip having a drivercircuit for driving the gate and signal lines by bonding or the like,which causes an increase in manufacturing costs.

Further, another object of the present invention is to reduce contactresistance or the like between wirings that connect elements in order toachieve high-speed driving of the driver circuit. For example, highcontact resistance between a gate wiring and an upper wiring mightdistort an input signal.

Further, another object of the present invention is to provide astructure of a display device in which the number of contact holes andan area occupied by a driver circuit is reduced.

A pixel portion and a driver circuit for driving the pixel portion areformed over one substrate. At least a part of the driver circuit isformed using an inverted staggered thin film transistor in which anoxide semiconductor is used. The driver circuit as well as the pixelportion is provided over the same substrate, whereby manufacturing costsare reduced.

As an oxide semiconductor used in this specification, a thin film of amaterial represented by InMO₃ (ZnO)_(m) (m>0) is formed, and a thin filmtransistor in which the thin film is used as a semiconductor layer ismanufactured. Note that M denotes one or more of metal elements selectedfrom gallium (Ga), iron (Fe), nickel (Ni), manganese (Mn), and cobalt(Co). In addition to a case where only Ga is contained as M, there is acase where Ga and any of the above metal elements other than Ga, forexample, Ga and Ni or Ga and Fe are contained as M. Moreover, in theoxide semiconductor, in some cases, a transition metal element such asFe or Ni or an oxide of the transition metal is contained as an impurityelement in addition to a metal element contained as M. In thisspecification, this thin film is also referred to as an“In—Ga—Zn—O-based non-single-crystal film”.

Table 1 shows a typical example of measurement by inductively coupledplasma mass spectrometry (ICP-MS). An oxide semiconductor film ofInGa_(0.95)Zn_(0.41)O_(3.33) is obtained under Condition 1 where atarget in which the ratio of In₂O₃ to Ga₂O₃ and ZnO is 1:1:1 (the ratioof In to Ga and Zn being 1:1:0.5) is used and the flow rate of an argongas in a sputtering method is 40 sccm. In addition, an oxidesemiconductor film of InGa_(0.94)Zn_(0.40)O_(3.31) is obtained underCondition 2 where the flow rates of an argon gas and oxygen in asputtering method are 10 sccm and 5 sccm, respectively.

TABLE 1 Flow Ratio Composition (Atomic %) Ar/O₂ In Ga Zn O CompositionFormula 40/0 17.6 16.7 7.2 58.6 InGa_(0.95)Zn_(0.41)O_(3.33) 10/5 17.716.7 7 58.6 InGa_(0.94)Zn_(0.40)O_(3.31)

Further, Table 2 shows results of quantification performed usingRutherford backscattering spectrometry (RBS) instead of ICP-MS.

TABLE 2 Flow Ratio Composition (Atomic %) Ar/O₂ In Ga Zn O ArComposition Formula 40/0 17 15.8 7.5 59.4 0.3InGa_(0.93)Zn_(0.44)O_(3.49) 10/5 16 14.7 7.2 61.7 0.4InGa_(0.92)Zn_(0.45)O_(3.86)

According to the results of the measurement of the sample in Condition 1by RBS, the oxide semiconductor film is InGa_(0.93)Zn_(0.44)O_(3.49). Inaddition, according to the results of the measurement of the sample inCondition 2 by RBS, the oxide semiconductor film isInGa_(0.92)Zn_(0.45)O_(3.86).

An amorphous structure is observed in the In—Ga—Zn—O-basednon-single-crystal film by X-ray diffraction (XRD). Note that heattreatment is performed on the In—Ga—Zn—O-based non-single-crystal filmof the examined sample at 200 to 500° C., typically 300 to 400° C., for10 minutes to 100 minutes after the film is formed by a sputteringmethod. In addition, a thin film transistor having electriccharacteristics such as an on/off ratio of greater than or equal to 10⁹and a mobility of greater than or equal to 10 at a gate voltage of ±20 Vcan be manufactured.

It is effective to use a thin film transistor having such electriccharacteristics for a driver circuit. For example, a gate line drivercircuit includes a shift register circuit for sequentially transferringgate signals, a buffer circuit, and the like; and a source line drivercircuit includes a shift register for sequentially transferring gatesignals, an analog switch for switching on and off of transfer of animage signal to a pixel, and the like. A TFT in which an oxidesemiconductor film is used has a higher mobility than a TFT in whichamorphous silicon is used and is capable of driving a shift registercircuit at high speed.

Further, when at least a part of a driver circuit for driving a pixelportion is formed using a thin film transistor in which an oxidesemiconductor is used, the TFTs included in the circuit are alln-channel TFTs, and a circuit illustrated in FIG. 1B is used as a basicunit. In addition, in the driver circuit, a gate electrode is directlyconnected to a source wiring or a drain wiring, whereby a favorablecontact can be obtained, which leads to a reduction in contactresistance. In the driver circuit, connecting the gate electrode to asource wiring or a drain wiring with another conductive film, e.g., atransparent conductive film might cause an increase in the number ofcontact holes, an increase in an area occupied by the contact holes dueto the increase in the number of contact holes, or an increase incontact resistance and wiring resistance, and might even complicate theprocess.

An embodiment of the present invention which is disclosed in thisspecification is a display device including a pixel portion and a drivercircuit. The pixel portion includes a first thin film transistorincluding at least a first oxide semiconductor layer. The driver circuitincludes a second thin film transistor including at least a second oxidesemiconductor layer and a third thin film transistor including a thirdoxide semiconductor layer. A wiring that is in direct contact with agate electrode of the second thin film transistor provided below thesecond oxide semiconductor layer is provided above the third oxidesemiconductor layer. The wiring is a source or drain wiring of the thirdthin film transistor which is electrically connected to the third oxidesemiconductor layer.

An embodiment of the present invention achieves at least one of theabove objects.

Further, the thin film transistor used for an embodiment of the presentinvention may include a fourth oxide semiconductor layer having smallerthickness and higher conductivity than the third oxide semiconductorlayer, between the source wiring and the oxide semiconductor layerserving as a channel formation region (the third semiconductor layer inthe above structure) or between the drain wiring and the oxidesemiconductor layer serving as the channel formation region (the thirdsemiconductor layer in the above structure).

The fourth oxide semiconductor layer exhibits n-type conductivity andfunctions as a source or drain region.

The third oxide semiconductor layer may have an amorphous structure, andthe fourth oxide semiconductor layer may include crystal grains(nanocrystals) in an amorphous structure. These crystal grains(nanocrystals) in the fourth oxide semiconductor layer each have adiameter of 1 to 10 nm, typically about 2 to 4 nm.

Further, as the fourth oxide semiconductor layer serving as a source ordrain region (an n⁺-type layer), an In—Ga—Zn—O-based non-single-crystalfilm can be used.

An insulating layer may be provided to cover the first thin filmtransistor, the second thin film transistor, and the third thin filmtransistor which are included in the display device and to be in contactwith the first oxide semiconductor layer, the second oxide semiconductorlayer, and the third oxide semiconductor layer. Further, in an etchingstep of the wiring, the oxide semiconductor layer may be partly etched.In that case, the first oxide semiconductor layer, the second oxidesemiconductor layer, and the third oxide semiconductor layer eachinclude a region with a small thickness.

Further, since the thin film transistor is easily broken by staticelectricity and the like, a protection circuit for protecting the drivercircuits is preferably provided over the same substrate for a gate lineor a source line. The protection circuit is preferably formed using anonlinear element in which an oxide semiconductor is used.

Note that ordinal numbers such as “first” and “second” in thisspecification are used for convenience. Therefore, they do not denotethe order of steps, the stacking order of layers, and particular nameswhich specify the invention.

Moreover, as a display device including a driver circuit, alight-emitting display device in which a light-emitting element is usedand a display device in which an electrophoretic display element isused, which is also referred to as an “electronic paper”, are given inaddition to a liquid crystal display device.

In the light-emitting display device in which a light-emitting elementis used, a plurality of thin film transistors are included in a pixelportion, and also in the pixel portion, there is a region where a gateelectrode of one thin film transistor is directly connected to a sourcewiring or a drain wiring of another transistor. In addition, in thedriver circuit of the light-emitting display device in which alight-emitting element is used, there is a region where a gate electrodeof a thin film transistor is directly connected to a source wiring or adrain wiring of the thin film transistor.

Note that the semiconductor device in this specification refers to allthe devices which can operate by using semiconductor characteristics,and an electro-optical device, a semiconductor circuit, and anelectronic appliance are all semiconductor devices.

A thin film transistor in which an oxide semiconductor is used in a gateline driver circuit or source line driver circuit, whereby manufacturingcosts can be reduced. Moreover, a gate electrode of the thin filmtransistor used for the driver circuit is directly connected to a sourcewiring or a drain wiring, whereby a display device in which the numberof contact holes can be reduced and an area occupied by the drivercircuit is reduced can be provided.

Therefore, according to an embodiment of the present invention, adisplay device having excellent electrical properties and highreliability can be provided at low costs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C illustrate a semiconductor device.

FIGS. 2A and 2B illustrate a semiconductor device.

FIGS. 3A to 3C illustrate a method for manufacturing a semiconductordevice.

FIGS. 4A to 4D illustrate a method for manufacturing a semiconductordevice.

FIGS. 5A to 5C illustrate a method for manufacturing a semiconductordevice.

FIGS. 6A to 6C illustrate a method for manufacturing a semiconductordevice.

FIG. 7 illustrates a method for manufacturing a semiconductor device.

FIG. 8 illustrates a method for manufacturing a semiconductor device.

FIG. 9 illustrates a method for manufacturing a semiconductor device.

FIG. 10 illustrates a semiconductor device.

FIGS. 11A1 to 11B2 illustrate a semiconductor device.

FIG. 12 illustrates a semiconductor device.

FIG. 13 illustrates a semiconductor device.

FIGS. 14A and 14B are block diagrams each illustrating a semiconductordevice.

FIG. 15 illustrates a configuration of a signal line driver circuit.

FIG. 16 is a timing chart illustrating operation of a signal line drivercircuit.

FIG. 17 is a timing chart illustrating operation of a signal line drivercircuit.

FIG. 18 illustrates a configuration of a shift register.

FIG. 19 illustrates a connection structure of the flip-flop illustratedin FIG. 18.

FIG. 20 illustrates a pixel equivalent circuit of a semiconductordevice.

FIGS. 21A to 21C illustrate a semiconductor device.

FIGS. 22A1 to 22B illustrate a semiconductor device.

FIG. 23 illustrates a semiconductor device.

FIGS. 24A and 24B illustrate a semiconductor device.

FIGS. 25A and 25B each illustrate an example of a usage pattern of anelectronic paper.

FIG. 26 is an external view of an example of an electronic book reader.

FIG. 27A is an external view of an example of a television device andFIG. 27B is an external view of an example of a digital photo frame.

FIGS. 28A and 28B each illustrate an example of an amusement machine.

FIGS. 29A and 29B each illustrate an example of a mobile phone.

FIG. 30 illustrates a semiconductor device.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described in detail with reference to theaccompanying drawings. Note that the present invention is not limited tothe following description and it will be readily appreciated by thoseskilled in the art that modes and details can be modified in variousways without departing from the spirit and the scope of the presentinvention. Accordingly, the present invention should not be construed asbeing limited to the description of the embodiments to be given below.Note that in a structure of the present invention described below, likeportions or portions having like functions in different drawings aredenoted by the like reference numerals and repeated description thereofis omitted.

Embodiment 1

In this embodiment, an embodiment of the present invention will bedescribed based on an example in which an inverter circuit is formedusing two n-channel thin film transistors.

A driver circuit for driving a pixel portion is formed using an invertercircuit, a capacitor, a resistor, and the like. When two n-channel TFTsare combined to form an inverter circuit, there are two types ofcombinations: a combination of an enhancement type transistor and adepletion type transistor (hereinafter, a circuit formed by such acombination is referred to as an “EDMOS circuit”) and a combination ofenhancement type TFTs (hereinafter, a circuit formed by such acombination is referred to as an “EEMOS circuit”). Note that when thethreshold voltage of the n-channel TFT is positive, the n-channel TFT isdefined as an enhancement type transistor, while when the thresholdvoltage of the n-channel TFT is negative, the n-channel TFT is definedas a depletion type transistor, and this specification follows the abovedefinitions.

The pixel portion and the driver circuit are formed over the samesubstrate. In the pixel portion, on and off of voltage application to apixel electrode is switched using enhancement type transistors arrangedin a matrix. An oxide semiconductor is used for these enhancement typetransistors arranged in the pixel portion. Since the enhancement typetransistor has electric characteristics such as an on/off ratio ofgreater than or equal to 10⁹ at a gate voltage of ±20 V, leakage currentis small and low power consumption drive can be realized.

FIG. 1A illustrates a cross-sectional structure of the inverter circuitof the driver circuit. Note that a first thin film transistor 430 and asecond thin film transistor 431 which are illustrated in FIGS. 1A to 1Care each an inverted staggered thin film transistor and exemplifies athin film transistor in which a wiring is provided over a semiconductorlayer with a source or drain regions interposed therebetween.

In FIG. 1A, a first gate electrode 401 and a second gate electrode 402are provided over a substrate 400. The first gate electrode 401 and thesecond gate electrode 402 can be formed to have a single-layer structureor a stacked-layer structure using a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium, or an alloy material containing any of these materials as themain component.

For example, as a two-layer structure of each of the first gateelectrode 401 and the second gate electrode 402, the followingstructures are preferable: a two-layer structure of an aluminum layerand a molybdenum layer stacked thereover, a two-layer structure of acopper layer and a molybdenum layer stacked thereover, a two-layerstructure of a copper layer and a titanium nitride layer or a tantalumnitride layer stacked thereover, and a two-layer structure of a titaniumnitride layer and a molybdenum layer. As a three-layer structure, astack of a tungsten layer or a tungsten nitride layer, a layer of analloy of aluminum and silicon or an alloy of aluminum and titanium, anda titanium nitride layer or a titanium layer is preferable.

Further, over a gate insulating layer 403 covering the first gateelectrode 401 and the second gate electrode 402, a first oxidesemiconductor layer 405 and a second oxide semiconductor layer 407 areprovided.

Further, over the first oxide semiconductor layer 405, a first wiring409 and a second wiring 410 are provided. The second wiring 410 isdirectly connected to the second gate electrode 402 through a contacthole 404 formed in the gate insulating layer 403. In addition, a thirdwiring 411 is provided over the second oxide semiconductor layer 407.

The first thin film transistor 430 includes the first gate electrode 401and the first oxide semiconductor layer 405 overlapped with the firstgate electrode 401 with the gate insulating layer 403 interposedtherebetween, and the first wiring 409 is a power supply line at aground potential (a ground power supply line). This power supply line ata ground potential may be a power supply line to which a negativevoltage VDL is applied (a negative power supply line).

In addition, the second thin film transistor 431 includes the secondgate electrode 402 and the second oxide semiconductor layer 407overlapped with the second gate electrode 402 with the gate insulatinglayer 403 interposed therebetween. The third wiring 411 is a powersupply line to which a positive voltage VDD is applied (a positive powersupply line).

An n⁺-type layer 406 a is provided between the first oxide semiconductorlayer 405 and the first wiring 409, and an n⁺-type layer 406 b isprovided between the first oxide semiconductor layer 405 and the secondwiring 410. Further, an n⁺-type layer 408 a is provided between thesecond oxide semiconductor layer 407 and the second wiring 410, and ann⁺-type layer 408 b is provided between the second oxide semiconductorlayer 407 and the third wiring 411.

In this Embodiment, the n⁺-type layers 406 a, 406 b, 408 a, and 408 beach functioning as a source region or a drain region are formed ofIn—Ga—Zn—O-based non-single-crystal films, and deposition conditionsthereof are different from those of the first oxide semiconductor layer405 and the second oxide semiconductor layer 407. The n⁺-type layers 406a, 406 b, 408 a, and 408 b are oxide semiconductor layers having lowerresistance than the first oxide semiconductor layer 405 and the secondoxide semiconductor layer 407. For example, when formed of oxidesemiconductor films under Condition 1 shown in the above Table 1 where asputtering method is employed and the flow rate of an argon gas is 40sccm in a sputtering method, the n⁺-type layers 406 a, 406 b, 408 a, and408 b have n-type conductivity and an activation energy (ΔE) of from0.01 to 0.1 eV. Note that in this embodiment, the n⁺-type layers 406 a,406 b, 408 a, and 408 b are In—Ga—Zn—O-based non-single-crystal filmsand include at least an amorphous component. The n⁺-type layers 406 a,406 b, 408 a, and 408 b may include crystal grains (nanocrystals) in anamorphous structure. These crystal grains (nanocrystals) in the n⁺-typelayers 406 a, 406 b, 408 a, and 408 b each have a diameter of 1 to 10nm, typically about 2 to 4 nm.

By the provision of the n⁺-type layers 406 a, 406 b, 408 a, and 408 b,the first wiring 409 and the second wiring 410 which are metal layerscan have a good junction with the first oxide semiconductor layer 405,and the second wiring 410 and the third wiring 411 which are metallayers can have a good junction with the second oxide semiconductorlayer 407, so that stable operation can be realized in terms of heat incomparison with a Schottky junction. In addition, provision of then⁺-type layer is positively effective in supplying carriers to thechannel (on the source side), stably absorbing carriers from the channel(on the drain side), or preventing resistance from being generated at aninterface between the wiring and the oxide semiconductor layer.Moreover, since resistance is reduced, good mobility can be ensured evenwith a high drain voltage.

As illustrated in FIG. 1A, the second wiring 410 which is electricallyconnected to both the first oxide semiconductor layer 405 and the secondoxide semiconductor layer 407 is directly connected to the second gateelectrode 402 of the second thin film transistor 431 through the contacthole 404 formed in the gate insulating layer 403. By the directconnection, favorable contact can be obtained, which leads to areduction in contact resistance. In comparison with the case where thesecond gate electrode 402 and the second wiring 410 are connected toeach other with another conductive film, e.g., a transparent conductivefilm, a reduction in the number of contact holes and a reduction in anarea occupied by the driver circuit due to the reduction in the numberof contact holes can be achieved.

Further, FIG. 1C is a top view of the inverter circuit of the drivercircuit. In FIG. 1C, a cross section taken along the chain line Z1-Z2corresponds to FIG. 1A.

Further, FIG. 1B illustrates an equivalent circuit of the EDMOS circuit.The circuit connection illustrated in FIGS. 1A and 1C corresponds tothat illustrated in FIG. 1B. An example in which the first thin filmtransistor 430 is an enhancement type n-channel transistor and thesecond thin film transistor 431 is a depression type n-channeltransistor is illustrated.

In order to manufacture an enhancement type n-channel transistor and adepression type n-channel transistor over the same substrate, forexample, the first oxide semiconductor layer 405 and the secondsemiconductor layer 407 are formed using different materials or underdifferent conditions. Alternatively, an EDMOS circuit may be formed insuch a manner that gate electrodes are provided over and under the oxidesemiconductor layer to control the threshold value and a voltage isapplied to the gate electrodes so that one of the TFTs is normally onwhile the other TFT is normally off.

Embodiment 2

Although the example of the EDMOS circuit is described in Embodiment 1,an equivalent circuit of an EEMOS circuit is illustrated in FIG. 2A inthis embodiment. In the equivalent circuit illustrated in FIG. 2A, adriver circuit can be formed in either case: a case where a first thinfilm transistor 460 and a second thin film transistor 461 are bothenhancement type n-channel transistors, or a case where the first thinfilm transistor 460 is an enhancement type n-channel transistor and thesecond thin film transistor 461 is a depletion n-channel transistor.

It can be said that it is preferable to use the circuit configurationillustrated in FIG. 2A in which enhancement type n-channel transistorsof the same type are combined for the driver circuit. This is because inthat case, a transistor used for a pixel portion is also formed of anenhancement type n-channel transistor which is the same type as thatused for the driver circuit, and therefore the number of manufacturingsteps is not increased. FIG. 2B is a top view. In FIG. 2B, a crosssection taken along the chain line Y1-Y2 corresponds to FIG. 2A.

Note that the first thin film transistor 460 and the second thin filmtransistor 461 which are illustrated in FIGS. 2A and 2B are each aninversed staggered thin film transistor and exemplify a thin filmtransistor in which a wiring is formed over a semiconductor layer with asource region or a drain region interposed therebetween.

In addition, an example of a manufacturing process of an invertercircuit is illustrated in FIGS. 3A to 3C.

A first conductive film is formed over a substrate 440 by a sputteringmethod and the first conductive film is etched as selected using a firstphotomask to form a first gate electrode 441 and a second gate electrode442. Next, a gate insulating layer 443 for covering the first gateelectrode 401 and the second gate electrode 442 is formed by a plasmaCVD method or a sputtering method. The gate insulating layer 443 can beformed to have a single layer or a stack of a silicon oxide layer, asilicon nitride layer, a silicon oxynitride layer, or a silicon nitrideoxide layer by a CVD method, a sputtering method, or the like.Alternatively, the gate insulating layer 443 can be formed of a siliconoxide layer by a CVD method using an organosilane gas. As theorganosilane gas, a silicon-containing compound such astetraethoxysilane (TEOS: chemical formula, Si(OC₂H₅)₄),tetramethylsilane (TMS: chemical formula, Si(CH₃)₄),tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane(OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (SiH(OC₂H₅)₃), ortrisdimethylaminosilane (SiH(N(CH₃)₂)₃) can be used.

Next, the gate insulating layer 443 is etched as selected using a secondphotomask to form a contact hole 444 that reaches the second gateelectrode 442. A cross-sectional view of the steps so far corresponds toFIG. 3A.

Next, an oxide semiconductor film is formed by a sputtering method, andan n⁺-type layer is formed thereover. Note that reverse sputtering inwhich plasma is generated after introduction of an argon gas ispreferably performed to remove dust attached to a surface of the gateinsulating layer 443 and the bottom surface of the contact hole 444before the oxide semiconductor film is formed by a sputtering method.The reverse sputtering refers to a method in which, without applicationof a voltage to a target side, an RF power source is used forapplication of a voltage to a substrate side in an argon atmosphere tomodify a surface. Note that nitrogen, helium, or the like may be usedinstead of an argon atmosphere. Alternatively, the reverse sputteringmay be performed in an argon atmosphere to which oxygen, hydrogen, N₂O,or the like is added. Further alternatively, the reverse sputtering maybe performed in an argon atmosphere to which Cl₂, CF₄, or the like isadded.

Next, the oxide semiconductor film and the n⁺-type layer are etched asselected using a third photomask. Then, a second conductive film isformed by a sputtering method and the second conductive film is etchedas selected using a fourth photomask to form a first wiring 449, asecond wiring 450, and a third wiring 451. The third wiring 451 isdirectly in contact with the second gate electrode 442 through thecontact hole 444. Note that reverse sputtering in which plasma isgenerated after introduction of an argon gas is preferably performed toremove dust attached to a surface of the gate insulating layer 443, asurface of the n⁺-type layer, and the bottom surface of the contact hole444 before the second conductive film is formed by a sputtering method.The reverse sputtering refers to a method in which, without applicationof a voltage to a target side, an RF power source is used forapplication of a voltage to a substrate side in an argon atmosphere tomodify a surface. Note that nitrogen, helium, or the like may be usedinstead of an argon atmosphere. Alternatively, the reverse sputteringmay be performed in an argon atmosphere to which oxygen, hydrogen, N₂O,or the like is added. Further alternatively, the reverse sputtering maybe performed in an argon atmosphere to which Cl₂, CF₄, or the like isadded.

Note that in the etching of the second conductive film, parts of then⁺-type layers and the oxide semiconductor films are also etched to formn⁺-type layers 446 a, 446 b, 448 a, and 448 b and a first oxidesemiconductor layer 445 and a second oxide semiconductor layer 447. Thisetching reduces the thickness of parts of the first oxide semiconductorlayer 445 and the second oxide semiconductor layer 447 so that the partsoverlapping with the first and second gate electrodes are thinned. Whenthis etching step is finished, the first thin film transistor 460 andthe second thin film transistor 461 are completed. A cross-sectionalview of the steps so far corresponds to FIG. 3B.

Next, heat treatment is performed at 200 to 600° C. in an air atmosphereor a nitrogen atmosphere. Note that the timing of this heat treatment isnot particularly limited and the heat treatment may be performed anytimeas long as it is performed after the formation of the oxidesemiconductor film.

Next, a protective layer 452 is formed and the protective layer 452 isetched as selected using a fifth photomask to form a contact hole. Afterthat, a third conductive film is formed. Lastly, the third conductivefilm is etched as selected using a sixth photomask to form a connectionwiring 453 that is electrically connected to the second wiring 410. Across-sectional view of the steps so far corresponds to FIG. 3C.

In a light-emitting display device in which a light-emitting element isused, a pixel portion has a plurality of thin film transistors, and thepixel portion also has a contact hole for electrically connecting a gateelectrode of one thin film transistor to a source wiring or a drainwiring of another transistor. This contact portion can be formed usingthe same mask as in the step of forming the contact hole in the gateinsulating layer using the second photomask.

Further, as for a liquid crystal display device or an electronic paper,in a terminal portion for connection to an external terminal such as anFPC, a contact hole that reaches a gate wiring can be formed using thesame mask as in a step of forming a contact hole in a gate insulatinglayer using the second photomask.

Note that the order of the steps described above is merely an exampleand there is no limitation to the order. For example, although thenumber of photomasks increases by one, etching of the second conductivefilm and etching of parts of the n⁺-type layers and the oxidesemiconductor films may be separately performed using differentphotomasks.

Embodiment 3

In this embodiment, an example of a manufacturing process of an invertercircuit which is different from the process described in Embodiment 2will be described using FIGS. 4A to 4C.

A first conductive film is formed over the substrate 440 by a sputteringmethod and the first conductive film is etched as selected using a firstphotomask to form the first gate electrode 441 and the second gateelectrode 442. Next, the gate insulating layer 443 for covering thefirst gate electrode 441 and the second gate electrode 442 is formed bya plasma CVD method or a sputtering method.

Next, an oxide semiconductor film is formed by a sputtering method, andan n⁺-type layer is formed thereover.

Next, the oxide semiconductor film and the n⁺-type layer are etched asselected using the second photomask. Thus, an oxide semiconductor film454 and an n⁺-type layer 455 are formed which are overlapped with thefirst gate electrode 441 with the gate insulating layer 443 interposedtherebetween. In addition, an oxide semiconductor film 456 and ann⁺-type layer 457 are formed which are overlapped with the second gateelectrode 442 with the gate insulating layer 443 interposedtherebetween. A cross-sectional view of the steps so far is illustratedin FIG. 4A.

Next, the gate insulating layer 443 is etched as selected using a thirdphotomask to form the contact hole 444 that reaches the second gateelectrode 442. A cross-sectional view of the steps so far corresponds toFIG. 4B.

Next, the second conductive film is formed by a sputtering method andetched as selected using a fourth photomask to form the first wiring449, the second wiring 450, and the third wiring 451. Note that reversesputtering in which plasma is generated after introduction of an argongas is preferably performed to remove dust attached to a surface of thegate insulating layer 443, a surface of the n⁺-type layers 455 and 457,and the bottom surface of the contact hole 444 before the secondconductive film is formed by a sputtering method. The reverse sputteringrefers to a method in which, without application of a voltage to atarget side, an RF power source is used for application of a voltage toa substrate side in an argon atmosphere to modify a surface. Note thatnitrogen, helium, or the like may be used instead of an argonatmosphere. Alternatively, the reverse sputtering may be performed in anargon atmosphere to which oxygen, hydrogen, N₂O, or the like is added.Further alternatively, the reverse sputtering may be performed in anargon atmosphere to which Cl₂, CF₄, or the like is added.

In the process of this embodiment, since the second conductive film canbe formed without formation of any other film after the contact hole 444is formed, the number of steps involving exposure of the bottom surfaceof the contact hole is smaller than that in Embodiment 2; therefore, amaterial for the gate electrode can be chosen from a wider range. InEmbodiment 2, since the oxide semiconductor film is formed in contactwith the gate electrode surface exposed in the contact hole 444, etchingconditions or a material of the gate electrode should be selected sothat the material of the gate electrode is not etched through the stepof etching the oxide semiconductor film.

Note that in the etching of the second conductive film, parts of then⁺-type layers and the oxide semiconductor films are also etched to formthe n⁺-type layers 446 a, 446 b, 448 a, and 448 b and the first oxidesemiconductor layer 445 and the second oxide semiconductor layer 447.This etching reduces the thickness of parts of the first oxidesemiconductor layer 445 and the second oxide semiconductor layer 447 sothat the parts overlapping with the first and second gate electrodes arethinned. When this etching step is finished, the first thin filmtransistor 460 and the second thin film transistor 461 are completed.

The first thin film transistor 460 includes the first gate electrode 441and the first oxide semiconductor layer 445 overlapped with the firstgate electrode 441 with the gate insulating layer 443 interposedtherebetween, and the first wiring 449 is a power supply line at aground potential (a ground power supply line). This power supply line ata ground potential may be a power supply line to which a negativevoltage VDL is applied (a negative power supply line).

In addition, the second thin film transistor 461 includes the secondgate electrode 442 and the second oxide semiconductor layer 447overlapped with the second gate electrode 442 with the gate insulatinglayer 443 interposed therebetween. The third wiring 451 is a powersupply line to which a positive voltage VDD is applied (a positive powersupply line).

The n⁺-type layer 446 a is provided between the first oxidesemiconductor layer 445 and the first wiring 449, and the n⁺-type layer446 b is provided between the first oxide semiconductor layer 445 andthe second wiring 450. Further, the n⁺-type layer 448 a is providedbetween the second oxide semiconductor layer 447 and the second wiring450, and the n⁺-type layer 448 b is provided between the second oxidesemiconductor layer 447 and the third wiring 451.

A cross-sectional view of the steps so far corresponds to FIG. 4C.

Next, heat treatment is performed at 200 to 600° C. in an air atmosphereor a nitrogen atmosphere. Note that the timing of this heat treatment isnot particularly limited and the heat treatment may be performed anytimeas long as it is performed after the formation of the oxidesemiconductor film.

Next, the protective layer 452 is formed and the protective layer 452 isetched as selected using a fifth photomask to form a contact hole. Afterthat, a third conductive film is formed. Lastly, the third conductivefilm is etched as selected using a sixth photomask to form theconnection wiring 453 that is electrically connected to the secondwiring 450.

In a light-emitting display device in which a light-emitting element isused, a pixel portion has a plurality of thin film transistors, and thepixel portion also has a contact hole for electrically connecting a gateelectrode of one thin film transistor to a source wiring or a drainwiring of another transistor. This contact portion can be formed usingthe same mask as in the step of forming the contact hole in the gateinsulating layer using the third photomask.

Further, as for a liquid crystal display device or an electronic paper,in a terminal portion for connection to an external terminal such as anFPC, a contact hole that reaches a gate wiring can be formed using thesame mask as in a step of forming a contact hole in a gate insulatinglayer using the third photomask.

Note that the order of the steps described above is merely an exampleand there is no limitation to the order. For example, although thenumber of photomasks increases by one, etching of the second conductivefilm and etching of parts of the n⁺-type layers and the oxidesemiconductor films may be separately performed using differentphotomasks.

Embodiment 4

In this embodiment, a manufacturing process of a display deviceincluding a thin film transistor according to an embodiment of thepresent invention will be described using FIGS. 5A to 5C, FIGS. 6A to6C, FIG. 7, FIG. 8, FIG. 9, FIG. 10, FIGS. 11A1 to 11B2, and FIG. 12.

In FIG. 5A, as a substrate 100 having a light-transmitting property, aglass substrate of barium borosilicate glass, aluminoborosilicate glass,or the like typified by #7059 glass, #1737 glass, or the likemanufactured by Corning, Inc. can be used.

Next, a conductive layer is formed over the entire surface of thesubstrate 100. After that, a first photolithography step is performed toform a resist mask, and unnecessary portions are removed by etching,thereby forming wirings and an electrode (a gate wiring including a gateelectrode layer 101, a capacitor wiring 108, and a first terminal 121).At this time, the etching is performed so that at least end portions ofthe gate electrode layer 101 have a tapered shape. A cross-sectionalview at this stage is illustrated in FIG. 5A. Note that a top view atthis stage corresponds to FIG. 7.

The gate wiring including the gate electrode layer 101, the capacitorwiring 108, and the first terminal 121 of a terminal portion arepreferably formed from a conductive material having low resistance, suchas aluminum (Al) or copper (Cu). However, since use of Al alone bringsdisadvantages such as low resistance and a tendency to be corroded,aluminum is used in combination with a conductive material having heatresistance. As the conductive material having heat resistance, any ofthe following materials may be used: an element selected from titanium(Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), andneodymium (Nd), scandium (Sc), an alloy containing any of these elementsas a component, an alloy containing any of these elements incombination, and a nitride containing any of these elements as acomponent.

Next, a gate insulating layer 102 is formed over the entire surface ofthe gate electrode layer 101. The gate insulating layer 102 is formed toa thickness of 50 to 250 nm by a sputtering method or the like.

For example, for the gate insulating layer 102, a 100-nm-thick siliconoxide film is formed by a sputtering method. Needless to say, the gateinsulating layer 102 is not limited to such a film and may be a singlelayer or a stack of any other types of insulating films such as asilicon oxynitride film, a silicon nitride film, an aluminum oxide film,and a tantalum oxide film.

Note that reverse sputtering in which plasma is generated afterintroduction of an argon gas is preferably performed to remove dustattached to a surface of the gate insulating layer before the oxidesemiconductor film is formed. Note that nitrogen, helium, or the likemay be used instead of an argon atmosphere. Alternatively, the reversesputtering may be performed in an argon atmosphere to which oxygen,hydrogen, N₂O, or the like is added. Further alternatively, the reversesputtering may be performed in an argon atmosphere to which Cl₂, CF₄, orthe like is added.

Next, a first oxide semiconductor film (in this embodiment, a firstIn—Ga—Zn—O-based non-single-crystal film) is formed over the gateinsulating layer 102. Formation of the first In—Ga—Zn—O-basednon-single-crystal film without exposure to air after the plasmatreatment is effective in preventing dust and moisture from attaching tothe interface between the gate insulating layer and the semiconductorfilm. Here, the first In—Ga—Zn—O-based non-single-crystal film is formedin an argon or oxygen atmosphere using an oxide semiconductor targethaving a diameter of 8 inches and containing In, Ga, and Zn (the ratioof In₂O₃ to Ga₂O₃ and ZnO is 1:1:1), with the distance between thesubstrate and the target set to 170 mm, under a pressure of 0.4 Pa, andwith a direct-current (DC) power source of 0.5 kW. Note that it ispreferable to use a pulsed direct-current (DC) power source with whichdust can be reduced and thickness distribution can be evened. The firstIn—Ga—Zn—O-based non-single-crystal film has a thickness of 5 to 200 nm.In this embodiment, the thickness of the first In—Ga—Zn—O-basednon-single-crystal film is 100 nm.

Next, a second oxide semiconductor film (in this embodiment, a secondIn—Ga—Zn—O-based non-single-crystal film) is formed by a sputteringmethod without exposure to air. Here, sputtering is performed using atarget in which the ratio of In₂O₃ to Ga₂O₃ and ZnO is 1:1:1 underdeposition conditions where the pressure is 0.4 Pa, the power is 500 W,the deposition temperature is room temperature, and an argon gas isintroduced at a flow rate of 40 sccm. Despite the intentional use of thetarget in which the ratio of In₂O₃ to Ga₂O₃ and ZnO is 1:1:1, anIn—Ga—Zn—O-based non-single-crystal film including crystal grains with asize of 1 to 10 nm immediately after the film formation may be formed.Note that it can be said that the presence or absence of crystal grainsor the density of crystal grains can be adjusted and the diameter sizecan be adjusted within the range of 1 to 10 nm by appropriate adjustmentof the composition ratio in the target, the deposition pressure (0.1 to2.0 Pa), the power (250 to 3000 W: 8 inches ø), the temperature (roomtemperature to 100° C.), the reactive sputtering conditions fordeposition, or the like. The second In—Ga—Zn—O-based non-single-crystalfilm has a thickness of 5 to 20 nm. Needless to say, when the filmincludes crystal grains, the size of the crystal grains does not exceedthe thickness of the film. In this embodiment, the thickness of thesecond In—Ga—Zn—O-based non-single-crystal film is 5 nm.

The first In—Ga—Zn—O-based non-single-crystal film is formed underdeposition conditions different from deposition conditions for thesecond In—Ga—Zn—O-based non-single-crystal film. For example, the firstIn—Ga—Zn—O-based non-single-crystal film is formed under conditionswhere the ratio of an oxygen gas flow rate to an argon gas flow rate ishigher than the ratio of an oxygen gas flow rate to an argon gas flowrate under the deposition conditions for the second In—Ga—Zn—O-basednon-single-crystal film. Specifically, the second In—Ga—Zn—O-basednon-single-crystal film is formed in a rare gas (e.g., argon or helium)atmosphere (or an atmosphere, less than or equal to 10% of which is anoxygen gas and greater than or equal to 90% of which is an argon gas),and the first In—Ga—Zn—O-based non-single-crystal film is formed in anoxygen atmosphere (or an atmosphere in which the ratio of an argon gasflow rate to an oxygen gas flow rate is 1:1 or more).

A chamber used for deposition of the second In—Ga—Zn—O-basednon-single-crystal film may be the same or different from the chamber inwhich the reverse sputtering has been performed.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used as a sputtering power source, a DCsputtering method, and a pulsed DC sputtering method in which a bias isapplied in a pulsed manner. An RF sputtering method is mainly used inthe case where an insulating film is formed, and a DC sputtering methodis mainly used in the case where a metal film is formed.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

In addition, there are a sputtering apparatus provided with a magnetsystem inside the chamber and used for a magnetron sputtering, and asputtering apparatus used for an ECR sputtering in which plasmagenerated with the use of microwaves is used without using glowdischarge.

Furthermore, as a deposition method by sputtering, there are also areactive sputtering method in which a target substance and a sputteringgas component are chemically reacted with each other during depositionto form a thin film of a compound thereof, and a bias sputtering inwhich a voltage is also applied to a substrate during deposition.

Next, a second photolithography step is performed to form a resist mask,and the first In—Ga—Zn—O-based non-single-crystal film and the secondIn—Ga—Zn—O-based non-single-crystal film are etched. Here, by wetetching using ITO-07N (manufactured by KANTO CHEMICAL CO., INC.),unnecessary portions are removed by etching, thereby forming an oxidesemiconductor film 109 which is a first In—Ga—Zn—O-basednon-single-crystal film and an oxide semiconductor film 111 which is asecond In—Ga—Zn—O-based non-single-crystal film. Note that etching hereis not limited to wet etching and may be dry etching. A cross-sectionalview at this stage is illustrated in FIG. 5B. Note that a top view atthis stage corresponds to FIG. 8.

Next, a third photolithography step is performed to form a resist mask,and unnecessary portions are removed by etching, thereby forming acontact hole reaching the wiring or the electrode layer which are formedfrom the same material as the gate electrode layer. This contact hole isprovided for direct contact with the conductive film formed later. Forexample, a contact hole is formed when a thin film transistor whose gateelectrode layer is in direct contact with the source or drain electrodelayer is formed in the driving circuit, or when a terminal that iselectrically connected to a gate wiring of a terminal portion is formed.

Next, over the oxide semiconductor film 109 and the oxide semiconductorfilm 111, a conductive film 132 formed from a metal material is formedby a sputtering method or a vacuum evaporation method. FIG. 5C is across-sectional view at this stage.

As the material of the conductive film 132, there are an elementselected from Al, Cr, Ta, Ti, Mo, and W, an alloy containing any ofthese elements as a component, an alloy containing these elements incombination, and the like. Further, for heat treatment at 200 to 600°C., the conductive film preferably has heat resistance for such heattreatment. Since use of Al alone brings disadvantages such as lowresistance and a tendency to be corroded, aluminum is used incombination with a conductive material having heat resistance. As theconductive material having heat resistance which is used in combinationwith Al, any of the following materials may be used: an element selectedfrom titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo),chromium (Cr), and neodymium (Nd), scandium (Sc), an alloy containingany of these elements as a component, an alloy containing these elementsin combination, and a nitride containing any of these elements as acomponent.

Here, the conductive film 132 has a single-layer structure of a titaniumfilm. Alternatively, the conductive film 132 may have a two-layerstructure: an aluminum film and a titanium film stacked thereon. Stillalternatively, the conductive film 132 may have a three-layer structure:a Ti film, an aluminum film containing Nd (Al—Nd) which is stacked onthe Ti film, and a Ti film formed on these films. The conductive film132 may have a single-layer structure of an aluminum film containingsilicon.

Next, a resist mask 131 is formed by a fourth photolithography step, andunnecessary portions are removed by etching, thereby forming source anddrain electrode layers 105 a and 105 b, n⁺-type layers 104 a and 104 bserving as source or drain regions and a connection electrode 120. Wetetching or dry etching is used as an etching method at this time. Forexample, when an aluminum film or an aluminum-alloy film is used as theconductive film 132, wet etching using a mixed solution of phosphoricacid, acetic acid, and nitric acid can be carried out. Here, by wetetching using an ammonia hydrogen peroxide mixture (with the ratio ofhydrogen peroxide to ammonia and water being 5:2:2), the conductive film132 of a Ti film is etched to form the source and drain electrode layers105 a and 105 b, and the oxide semiconductor film 111 is etched to formthe n⁺-type layers 104 a and 104 b. In this etching step, an exposedregion of the oxide semiconductor film 109 is partly etched to be asemiconductor layer 103. Thus, a channel region of the semiconductorlayer 103 has a small thickness. The etching for forming the source anddrain electrode layers 105 a and 105 b and the n⁺-type layers 104 a and104 b is performed at a time using an etchant of an ammonia hydrogenperoxide mixture. Accordingly, in FIG. 6A, an end portion of the sourceand drain electrode layer 105 a and an end portion of the source ordrain electrode layer 105 b are aligned with an end portion of then⁺-type layer 104 a and an end portion of the n⁺-type layer 104 b,respectively; thus, continuous end portions are formed. In addition, wetetching etches the layers isotropically, so that the end portions of thesource and drain electrode layers 105 a and 105 b are recessed from theresist mask 131. Through the aforementioned steps, a thin filmtransistor 170 which includes the semiconductor layer 103 as a channelformation region can be manufactured. A cross-sectional view at thisstage is illustrated in FIG. 6A. Note that FIG. 9 is a top view at thisstage.

Next, heat treatment is preferably performed at 200 to 600° C.,typically, 300 to 500° C. Here, heat treatment is performed in anitrogen atmosphere in a furnace at 350° C. for 1 hour. Through thisheat treatment, rearrangement at the atomic level occurs in theIn—Ga—Zn—O-based non-single-crystal film. Because strain energy whichinhibits carrier movement is released by the heat treatment, the heattreatment (including optical annealing) is important. Note that there isno particular limitation on the timing of heat treatment as long as itis performed after formation of the second In—Ga—Zn—O-basednon-single-crystal film, and for example, heat treatment may beperformed after formation of a pixel electrode.

Furthermore, the exposed channel formation region of the semiconductorlayer 103 may be subjected to oxygen radical treatment, so that anormally-off thin film transistor can be obtained. In addition, theradical treatment can repair damage due to the etching of thesemiconductor layer 103. The radical treatment is preferably performedin an atmosphere of O₂ or N₂O, and preferably an atmosphere of N₂, He,or Ar each containing oxygen. The radical treatment may also beperformed in an atmosphere in which Cl₂ or CF₄ is added to the aboveatmosphere. Note that the radical treatment is preferably performed withno bias applied.

In the fourth photolithography step, a second terminal 122 made from thesame material as the source and drain electrode layers 105 a and 105 bis also left in the terminal portion. Note that the second terminal 122is electrically connected to a source wiring (a source wiring includingthe source or drain electrode layers 105 a and 105 b).

In addition, in the terminal portion, the connection electrode 120 isdirectly connected to the first terminal 121 of the terminal portionthrough a contact hole formed in the gate insulating film. Note thatalthough not illustrated here, a source or drain wiring of the thin filmtransistor of the driver circuit is directly connected to the gateelectrode through the same steps as the above steps.

Further, by use of a resist mask having regions with plural thicknesses(typically, two different thicknesses) which is formed using amulti-tone mask, the number of resist masks can be reduced, resulting insimplified process and lower costs.

Next, the resist mask 131 is removed, and a protective insulating layer107 is formed to cover the thin film transistor 170. For the protectiveinsulating layer 107, a silicon nitride film, a silicon oxide film, asilicon oxynitride film, an aluminum oxide film, a tantalum oxide film,or the like which is obtained by a sputtering method or the like can beused.

Then, a fifth photolithography step is performed to form a resist mask,and the protective insulating layer 107 is etched to form a contact hole125 which reaches the source or drain electrode layers 105 a or 105 b.In addition, by the etching here, a contact hole 127 which reaches thesecond terminal 122 and a contact hole 126 which reaches the connectionelectrode 120 are formed. A cross-sectional view at this stage isillustrated in FIG. 6B.

Then, after the resist mask is removed, a transparent conductive film isformed. The transparent conductive film is formed using indium oxide(In₂O₃), an alloy of indium oxide and tin oxide (In₂O₃—SnO₂, abbreviatedas ITO), or the like by a sputtering method, a vacuum evaporationmethod, or the like. Etching treatment of such a material is performedwith a hydrochloric acid based solution. Instead, because a residuetends to be generated particularly in etching of ITO, an alloy of indiumoxide and zinc oxide (In₂O₃—ZnO) may be used in order to improve etchingprocessability.

Next, a sixth photolithography step is performed to form a resist mask,and unnecessary portions are removed by etching, thereby forming a pixelelectrode layer 110.

Further, in this sixth photolithography step, a storage capacitor isformed with the capacitor wiring 108 and the pixel electrode layer 110.The storage capacitor includes the gate insulating layer 102 and theprotective insulating layer 107 in the capacitor portion as dielectrics.

In addition, in this sixth photolithography step, the first terminal andthe second terminal are covered with the resist mask, and transparentconductive films 128 and 129 are left in the terminal portion. Thetransparent conductive films 128 and 129 serve as electrodes or wiringsthat are used for connection with an FPC. The transparent conductivefilm 128 formed over the connection electrode 120 that is directlyconnected to the first terminal 121 serves as a terminal electrode forconnection which serves as an input terminal for the gate wiring. Thetransparent conductive film 129 formed over the second terminal 122serves as a terminal electrode for connection which serves as an inputterminal for the source wiring.

Then, the resist mask is removed, and a cross-sectional view at thisstage is illustrated in FIG. 6C. Note that a top view at this stagecorresponds to FIG. 10.

Further, FIGS. 11A1 and 11A2 are a cross-sectional view of a gate wiringterminal portion at this stage and a top view thereof, respectively.FIG. 11A1 is a cross-sectional view taken along the line C1-C2 of FIG.11A2. In FIG. 11A1, a transparent conductive film 155 formed over aprotective insulating film 154 is a connection terminal electrode whichfunctions as an input terminal. Furthermore, in FIG. 11A1, in theterminal portion, a first terminal 151 formed from the same material asthe gate wiring and a connection electrode 153 formed from the samematerial as the source wiring overlap with each other and are in directcontact and electrically connected through a contact hole provided in agate insulating layer 152. In addition, the connection electrode 153 andthe transparent conductive film 155 are in direct contact andelectrically connected with each other through a contact hole providedin the protective insulating film 154.

Further, FIGS. 11B1 and 11B2 are a cross-sectional view of a sourcewiring terminal portion at this stage and a top view thereof,respectively. In addition, FIG. 11B1 corresponds to a cross-sectionalview taken along the line D1-D2 in FIG. 11B2. In FIG. 11B1, thetransparent conductive film 155 formed over the protective insulatingfilm 154 is a connection terminal electrode which functions as an inputterminal. Furthermore, in FIG. 11B1, in the terminal portion, anelectrode 156 formed from the same material as the gate wiring islocated below and overlapped with a second terminal 150, which iselectrically connected to the source wiring, with the gate insulatinglayer 152 interposed therebetween. The electrode 156 is not electricallyconnected to the second terminal 150. When the electrode 156 is set to,for example, floating, GND, or 0 V such that the potential the electrode156 is different from the potential of the second terminal 150, acapacitor for preventing noise or static electricity can be formed. Inaddition, the second terminal 150 is electrically connected to thetransparent conductive film 155 with the protective insulating film 154interposed therebetween.

A plurality of gate wirings, source wirings, and capacitor wirings areprovided depending on the pixel density. Also in the terminal portion,the first terminal at the same potential as the gate wiring, the secondterminal at the same potential as the source wiring, the third terminalat the same potential as the capacitor wiring, and the like are eacharranged in plurality. There is no particular limitation on the numberof terminals, and the number of terminals may be determined by apractitioner as appropriate.

Through these six photolithography steps, a pixel thin film transistorportion including the thin film transistor 170, which is a bottom-gaten-channel thin film transistor, and the storage capacitor can becompleted using the six photomasks. When these pixel thin filmtransistor portion and storage capacitor are arranged in a matrixcorresponding to respective pixels, a pixel portion can be formed andone of the boards for manufacturing an active matrix display device canbe obtained. In this specification, such a board is referred to as anactive matrix substrate for convenience.

When an active matrix liquid crystal display device is manufactured, anactive matrix substrate and a counter substrate provided with a counterelectrode are bonded to each other with a liquid crystal layerinterposed therebetween. Note that a common electrode electricallyconnected to the counter electrode on the counter substrate is providedover the active matrix substrate, and a fourth terminal electricallyconnected to the common electrode is provided in the terminal portion.This fourth terminal is provided so that the common electrode is fixedto a predetermined potential such as GND or 0 V.

Further, an embodiment of the present invention is not limited to apixel structure in FIG. 10, and an example of a top view different fromFIG. 10 is illustrated in FIG. 12. FIG. 12 illustrates an example inwhich a capacitor wiring is not provided and a storage capacitor isformed with a pixel electrode and a gate wiring of an adjacent pixelwhich are overlapped with each other with a protective insulating filmand a gate insulating layer interposed therebetween. In this case, thecapacitor wiring and the third terminal connected to the capacitorwiring can be omitted. Note that in FIG. 12, portions similar to thosein FIG. 10 are denoted by the same reference numerals.

In an active matrix liquid crystal display device, pixel electrodesarranged in a matrix are driven to form a display pattern on a screen.Specifically, a voltage is applied between a selected pixel electrodeand a counter electrode corresponding to the selected pixel electrode,so that a liquid crystal layer provided between the pixel electrode andthe counter electrode is optically modulated and this optical modulationis recognized as a display pattern by an observer.

In displaying moving images, a liquid crystal display device has aproblem that a long response time of liquid crystal molecules causesafterimages or blurring of moving images. In order to improve themoving-image characteristics of a liquid crystal display device, adriving method called black insertion is employed in which black isdisplayed on the whole screen every other frame period.

Alternatively, a driving method called double-frame rate driving may beemployed in which the vertical cycle is 1.5 or 2 times as long as usualto improve the moving-image characteristics.

Further alternatively, in order to improve the moving-imagecharacteristics of a liquid crystal display device, a driving method maybe employed in which a plurality of LEDs (light-emitting diodes) or aplurality of EL light sources are used to form a surface light source asa backlight, and each light source of the surface light source isindependently driven in a pulsed manner in one frame period. As thesurface light source, three or more kinds of LEDs may be used and an LEDemitting white light may be used. Since a plurality of LEDs can becontrolled independently, the light emission timing of LEDs can besynchronized with the timing at which a liquid crystal layer isoptically modulated. According to this driving method, LEDs can bepartly turned off; therefore, an effect of reducing power consumptioncan be obtained particularly in the case of displaying an image having alarge black part.

By combining these driving methods, the display characteristics of aliquid crystal display device, such as moving-image characteristics, canbe improved as compared to those of conventional liquid crystal displaydevices.

The n-channel transistor obtained in this embodiment includes anIn—Ga—Zn—O-based non-single-crystal film in a channel formation regionand has good dynamic characteristics. Thus, these driving methods can beapplied in combination.

When a light-emitting display device is manufactured, one electrode(also referred to as a cathode) of an organic light-emitting element isset to a low power supply potential such as GND or 0 V; therefore, aterminal portion is provided with a fourth terminal for setting thecathode to a low power supply potential such as GND or 0 V. In addition,when a light-emitting display device is manufactured, a power supplyline is provided in addition to a source wiring and a gate wiring.Therefore, a terminal portion is provided with a fifth terminalelectrically connected to the power supply line.

Thin film transistors in which an oxide semiconductor is used areprovided in a gate line driver circuit or a source line driver circuit,whereby manufacturing costs are reduced. Moreover, a gate electrode ofthe thin film transistor used for the driver circuit is directlyconnected to a source wiring or a drain wiring, whereby a display devicein which the number of contact holes can be reduced and an area occupiedby the driver circuit is reduced can be provided.

Therefore, according to an embodiment of the present invention, adisplay device having high electrical properties and high reliabilitycan be provided at low costs.

Embodiment 5

Here, an example of a display device including a thin film transistoraccording to Embodiment 1 in which a wiring is in contact with asemiconductor layer will be described with reference to FIG. 30.

FIG. 30 illustrates a cross-sectional structure of an inverter circuitof a driver circuit. Note that a first thin film transistor 480 and asecond thin film transistor 481 which are illustrated in FIG. 30 areeach an inverted staggered thin film transistor. The first wiring 409,and the second wiring 410 are provided in contact with the first oxidesemiconductor layer 405, and the second wiring 410, and the third wiring411 are provided in contact with the second oxide semiconductor layer407.

In the first thin film transistor 480 and the second thin filmtransistor 481, the following regions are preferably modified by plasmatreatment: a region where the first oxide semiconductor layer 405 is incontact with the first wiring 409, a region where the first oxidesemiconductor layer 405 is in contact with the second wiring 410, aregion where the second oxide semiconductor layer 407 is in contact withthe second wiring 410, and a region where the second oxide semiconductorlayer 407 is in contact with the third wiring 411. In this embodiment,before a conductive film serving as wirings is formed, an oxidesemiconductor layer (in this embodiment, an In—Ga—Zn—O-basednon-single-crystal film) is subjected to plasma treatment in an argonatmosphere.

For the plasma treatment, instead of an argon atmosphere, a nitrogenatmosphere, a helium atmosphere, or the like may be used. Alternatively,an argon atmosphere to which oxygen, hydrogen, N₂O, or the like is addedmay be used. Further alternatively, an argon atmosphere to which Cl₂,CF₄, or the like is added may be used.

In this embodiment, the first wiring 409, the second wiring 410, and thethird wiring 411 are made of a titanium film, and subjected to wetetching using an ammonia hydrogen peroxide mixture (with the ratio ofhydrogen peroxide to ammonia and water being 5:2:2). In this etchingstep, exposed regions of the semiconductor layer which isIn—Ga—Zn—O-based non-single-crystal film, are partly etched to be thefirst oxide semiconductor layer 405 and the second oxide semiconductorlayer 407. Thus, a channel region of the first semiconductor layer 405between the first wiring 409 and the second wiring 410 has a smallthickness. Similarly, a channel region of the second semiconductor layer407 between the second wiring 410 and the third wiring 411 has a smallthickness.

The conductive film is formed in contact with the first oxidesemiconductor layer 405 and the second oxide semiconductor layer 407which are modified by the plasma treatment; thus, the first wiring 409,the second wiring 410, and the third wiring 411 are formed. It ispossible to reduce the contact resistance between the first oxidesemiconductor layer 405 and the first wiring 409, the contact resistancebetween the first oxide semiconductor layer 405 and the second wiring410, the contact resistance between the second oxide semiconductor layer407 and the second wiring 410, and the contact resistance between thesecond oxide semiconductor layer 407 and the third wiring 411.

Through the above process, a highly reliable display device as asemiconductor device can be manufactured.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 6

This embodiment describes a display device which is an example of asemiconductor device according to an embodiment of the presentinvention. In that display device, at least a part of a driver circuitand a thin film transistor in a pixel portion are formed over onesubstrate.

The thin film transistor in the pixel portion is formed according toEmbodiment 4 or 5. The thin film transistor described in Embodiment 4 or5 is an n-channel TFT; therefore, part of a driver circuit which can beformed using n-channel TFTs is formed over the same substrate as thethin film transistor in the pixel portion.

FIG. 14A illustrates an example of a block diagram of an active matrixliquid crystal display device which is an example of a semiconductordevice according to an embodiment of the present invention. The displaydevice illustrated in FIG. 14A includes, over a substrate 5300, a pixelportion 5301 including a plurality of pixels each provided with adisplay element; a scan line driver circuit 5302 that selects a pixel;and a signal line driver circuit 5303 that controls a video signal inputto the selected pixel.

The pixel portion 5301 is connected to the signal line driver circuit5303 with a plurality of signal lines S1 to Sm (not shown) extending ina column direction from the signal line driver circuit 5303 andconnected to the scan line driver circuit 5302 with a plurality of scanlines G1 to Gn (not shown) extending in a row direction from the scanline driver circuit 5302. The pixel portion 5301 includes a plurality ofpixels (not shown) arranged in matrix corresponding to the signal linesS1 to Sm and the scan lines G1 to Gn. In addition, each of the pixels isconnected to a signal line Sj (any one of the signal lines S1 to Sm) anda scan line Gi (any one of the scan lines G1 to Gn).

The thin film transistor described in Embodiment 4 or 5 is an n-channelTFT. A signal line driver circuit including n-channel TFTs is describedwith reference to FIG. 15.

The signal line driver circuit of FIG. 15 includes a driver IC 5601,switch groups 5602_1 to 5602_M, a first wiring 5611, a second wiring5612, a third wiring 5613, and wirings 5621_1 to 5621_M. Each of theswitch groups 5602_1 to 5602_M includes a first thin film transistor5603 a, a second thin film transistor 5603 b, and a third thin filmtransistor 5603 c.

The driver IC 5601 is connected to the first wiring 5611, the secondwiring 5612, the third wiring 5613, and the wirings 5621_1 to 5621_M.Each of the switch groups 5602_1 to 5602_M is connected to the firstwiring 5611, the second wiring 5612, and the third wiring 5613. Inaddition, the switch groups 5602_1 to 5602_M are connected to thewirings 5621_1 to 5621_M, respectively. Each of the wirings 5621_1 to5621_M is connected to three signal lines through the first thin filmtransistor 5603 a, the second thin film transistor 5603 b, and the thirdthin film transistor 5603 c. For example, the wiring 5621_J of the J-thcolumn (one of the wirings 5621_1 to 5621_M) is connected to a signalline Sj−1, a signal line Sj, and a signal line Sj+1 through the firstthin film transistor 5603 a, the second thin film transistor 5603 b, andthe third thin film transistor 5603 c of the switch group 5602_J.

Note that a signal is input to each of the first wiring 5611, the secondwiring 5612, and the third wiring 5613.

Note that the driver IC 5601 is preferably formed on a single-crystalsubstrate. The switch groups 5602_1 to 5602_M are preferably formed overthe same substrate as the pixel portion. Therefore, the driver IC 5601is preferably connected to the switch groups 5602_1 to 5602_M through anFPC or the like.

Next, operation of the signal line driver circuit of FIG. 15 isdescribed with reference to a timing chart of FIG. 16. FIG. 16illustrates the timing chart where a scan line Gi in the i-th row isselected. A selection period of the scan line Gi in the i-th row isdivided into a first sub-selection period T1, a second sub-selectionperiod T2, and a third sub-selection period T3. In addition, the signalline driver circuit of FIG. 15 operates similarly to FIG. 16 when a scanline in another row is selected.

Note that the timing chart of FIG. 16 shows the case where the wiring5621_J in the J-th column is connected to the signal line Sj−1, thesignal line Sj, and the signal line Sj+1 through the first thin filmtransistor 5603 a, the second thin film transistor 5603 b, and the thirdthin film transistor 5603 c.

The timing chart of FIG. 16 shows timing when the scan line Gi in thei-th row is selected, timing 5703 a when the first thin film transistor5603 a is turned on/off, timing 5703 b when the second thin filmtransistor 5603 b is turned on/off, timing 5703 c when the third thinfilm transistor 5603 c is turned on/off, and a signal 5721_J input tothe wiring 5621_J in the J-th column.

In the first sub-selection period T1, the second sub-selection periodT2, and the third sub-selection period T3, different video signals areinput to the wirings 5621_1 to 5621_M. For example, a video signal inputto the wiring 5621_J in the first sub-selection period T1 is input tothe signal line Sj−1, a video signal input to the wiring 5621_J in thesecond sub-selection period T2 is input to the signal line Sj, and avideo signal input to the wiring 5621_J in the third sub-selectionperiod T3 is input to the signal line Sj+1. The video signals input tothe wiring 5621_J in the first sub-selection period T1, the secondsub-selection period T2, and the third sub-selection period T3 aredenoted by Data_j−1, Data_j, and Data_j+1, respectively.

As shown in FIG. 16, in the first sub-selection period T1, the firstthin film transistor 5603 a is on, and the second thin film transistor5603 b and the third thin film transistor 5603 c are off. At this time,Data_j−1 input to the wiring 5621_J is input to the signal line Sj−1through the first thin film transistor 5603 a. In the secondsub-selection period T2, the second thin film transistor 5603 b is on,and the first thin film transistor 5603 a and the third thin filmtransistor 5603 c are off. At this time, Data_j input to the wiring5621_J is input to the signal line Sj through the second thin filmtransistor 5603 b. In the third sub-selection period T3, the third thinfilm transistor 5603 c is on, and the first thin film transistor 5603 aand the second thin film transistor 5603 b are off. At this time,Data_j+1 input to the wiring 5621_J is input to the signal line Sj+1through the third thin film transistor 5603 c.

As described above, in the signal line driver circuit of FIG. 15, onegate selection period is divided into three; thus, video signals can beinput to three signal lines from one wiring 5621 in one gate selectionperiod. Therefore, in the signal line driver circuit of FIG. 15, thenumber of connections between the substrate provided with the driver IC5601 and the substrate provided with the pixel portion can be reduced toapproximately one third the number of signal lines. When the number ofconnections is reduced to approximately one third the number of signallines, the reliability, yield, and the like of the signal line drivercircuit of FIG. 15 can be improved.

Note that there is no particular limitation on the arrangement, number,driving method, and the like of the thin film transistors, as long asone gate selection period is divided into a plurality of sub-selectionperiods and video signals are input to a plurality of signal lines fromone wiring in the respective sub-selection periods as shown in FIG. 15.

For example, when video signals are input to three or more signal linesfrom one wiring in the respective sub-selection periods, it is onlynecessary to add a thin film transistor and a wiring for controlling thethin film transistor. Note that when one gate selection period isdivided into four or more sub-selection periods, one sub-selectionperiod becomes short. Therefore, one gate selection period is preferablydivided into two or three sub-selection periods.

As another example, as shown in a timing chart of FIG. 17, one selectionperiod may be divided into a precharge period Tp, the firstsub-selection period T1, the second sub-selection period T2, and thethird sub-selection period T3. The timing chart of FIG. 17 shows timingwhen the scan line Gi in the i-th row is selected, timing 5803 a whenthe first thin film transistor 5603 a is turned on/off, timing 5803 bwhen the second thin film transistor 5603 b is turned on/off, timing5803 c when the third thin film transistor 5603 c is turned on/off, anda signal 5821_J input to the wiring 5621_J in the J-th column. As shownin FIG. 17, the first thin film transistor 5603 a, the second thin filmtransistor 5603 b, and the third thin film transistor 5603 c are on inthe precharge period Tp. At this time, a precharge voltage Vp input tothe wiring 5621_J is input to the signal line Sj−1, the signal line Sj,and the signal line Sj+1 through the first thin film transistor 5603 a,the second thin film transistor 5603 b, and the third thin filmtransistor 5603 c, respectively. In the first sub-selection period T1,the first thin film transistor 5603 a is on, and the second thin filmtransistor 5603 b and the third thin film transistor 5603 c are off. Atthis time, Data_j−1 input to the wiring 5621_J is input to the signalline Sj−1 through the first thin film transistor 5603 a. In the secondsub-selection period T2, the second thin film transistor 5603 b is on,and the first thin film transistor 5603 a and the third thin filmtransistor 5603 c are off. At this time, Data_j input to the wiring5621_J is input to the signal line Sj through the second thin filmtransistor 5603 b. In the third sub-selection period T3, the third thinfilm transistor 5603 c is on, and the first thin film transistor 5603 aand the second thin film transistor 5603 b are off. At this time,Data_j+1 input to the wiring 5621_J is input to the signal line Sj+1through the third thin film transistor 5603 c.

As described above, in the signal line driver circuit of FIG. 15, towhich the timing chart of FIG. 17 is applied, the signal line can beprecharged by providing the precharge period before the sub-selectionperiods. Thus, a video signal can be written to a pixel with high speed.Note that portions in FIG. 17 which are similar to those in FIG. 16 aredenoted by the same reference numerals, and detailed description of thesame portions or portions having similar functions is omitted.

Now, a constitution of the scan line driver circuit is described. Thescan line driver circuit includes a shift register and a buffer. Also,the scan line driver circuit may include a level shifter in some cases.In the scan line driver circuit, when a clock signal (CLK) and a startpulse signal (SP) are input to the shift register, a selection signal isproduced. The generated selection signal is buffered and amplified bythe buffer, and the resulting signal is supplied to a corresponding scanline. Gate electrodes of transistors in pixels in one line are connectedto the scan line. Further, since the transistors in the pixels in oneline have to be turned on at the same time, a buffer which can feed alarge amount of current is used.

An example of a shift register used as part of the scan line drivercircuit is described with reference to FIG. 18 and FIG. 19.

FIG. 18 illustrates a circuit configuration of the shift register. Theshift register shown in FIG. 18 includes a plurality of flip-flops,flip-flops 5701_1 to 5701 _(—) n. Further, the shift register operatesby input of a first clock signal, a second clock signal, a start pulsesignal, and a reset signal.

Connection relationships of the shift register of FIG. 18 are described.In the flip-flop 5701 _(—) i (one of the flip-flops 5701_1 to 5701 _(—)n) of the i-th stage in the shift register of FIG. 18, a first wiring5501 shown in FIG. 19 is connected to a seventh wiring 5717 _(—) i−1; asecond wiring 5502 shown in FIG. 19 is connected to a seventh wiring5717 _(—) i+1; a third wiring 5503 shown in FIG. 19 is connected to aseventh wiring 5717 _(—) i; and a sixth wiring 5506 shown in FIG. 19 isconnected to a fifth wiring 5715.

Further, a fourth wiring 5504 shown in FIG. 19 is connected to a secondwiring 5712 in flip-flops of odd-numbered stages, and is connected to athird wiring 5713 in flip-flops of even-numbered stages. A fifth wiring5505 shown in FIG. 19 is connected to a fourth wiring 5714.

Note that the first wiring 5501 shown in FIG. 19 of the flip-flop 5701_1of a first stage is connected to a first wiring 5711, and the secondwiring 5502 shown in FIG. 19 of the flip-flop 5701 _(—) n of an n-thstage is connected to a sixth wiring 5716.

The first wiring 5711, the second wiring 5712, the third wiring 5713,and the sixth wiring 5716 may be referred to as a first signal line, asecond signal line, a third signal line, and a fourth signal line,respectively. The fourth wiring 5714 and the fifth wiring 5715 may bereferred to as a first power supply line and a second power supply line,respectively.

FIG. 19 illustrates the detail of the flip-flop shown in FIG. 18. Aflip-flop shown in FIG. 19 includes a first thin film transistor 5571, asecond thin film transistor 5572, a third thin film transistor 5573, afourth thin film transistor 5574, a fifth thin film transistor 5575, asixth thin film transistor 5576, a seventh thin film transistor 5577,and an eighth thin film transistor 5578. Note that the first thin filmtransistor 5571, the second thin film transistor 5572, the third thinfilm transistor 5573, the fourth thin film transistor 5574, the fifththin film transistor 5575, the sixth thin film transistor 5576, theseventh thin film transistor 5577, and the eighth thin film transistor5578 are n-channel transistors, and are turned on when the gate-sourcevoltage (V_(gs)) exceeds the threshold voltage (V_(th)).

In FIG. 19, a gate electrode of the third thin film transistor 5573 iselectrically connected to the power supply line. Further, it can be saidthat a circuit in which the third thin film transistor 5573 is connectedto the fourth thin film transistor 5574 (a circuit surrounded by thechain line in FIG. 19) corresponds to a circuit having the structureillustrated in FIG. 2A. Although the example in which all the thin filmtransistors are enhancement type n-channel transistors is describedhere, there is no limitation to this example. For example, the drivercircuit can be driven even with the use of a depletion type n-channeltransistor as the third thin film transistor 5573.

Now, a connection structure of the flip-flop shown in FIG. 19 isdescribed below.

A first electrode (one of a source electrode or a drain electrode) ofthe first thin film transistor 5571 is connected to the fourth wiring5504, and a second electrode (the other of the source electrode or thedrain electrode) of the first thin film transistor 5571 is connected tothe third wiring 5503.

A first electrode of the second thin film transistor 5572 is connectedto the sixth wiring 5506. A second electrode of the second thin filmtransistor 5572 is connected to the third wiring 5503.

A first electrode of the third thin film transistor 5573 is connected tothe fifth wiring 5505. A second electrode of the third thin filmtransistor 5573 is connected to a gate electrode of the second thin filmtransistor 5572. A gate electrode of the third thin film transistor 5573is connected to the fifth wiring 5505.

A first electrode of the fourth thin film transistor 5574 is connectedto the sixth wiring 5506. A second electrode of the fourth thin filmtransistor 5574 is connected to the gate electrode of the second thinfilm transistor 5572. A gate electrode of the fourth thin filmtransistor 5574 is connected to a gate electrode of the first thin filmtransistor 5571.

A first electrode of the fifth thin film transistor 5575 is connected tothe fifth wiring 5505. A second electrode of the fifth thin filmtransistor 5575 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the fifth thin film transistor5575 is connected to the first wiring 5501.

A first electrode of the sixth thin film transistor 5576 is connected tothe sixth wiring 5506. A second electrode of the sixth thin filmtransistor 5576 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the sixth thin film transistor5576 is connected to the gate electrode of the second thin filmtransistor 5572.

A first electrode of the seventh thin film transistor 5577 is connectedto the sixth wiring 5506. A second electrode of the seventh thin filmtransistor 5577 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the seventh thin filmtransistor 5577 is connected to the second wiring 5502. A firstelectrode of the eighth thin film transistor 5578 is connected to thesixth wiring 5506. A second electrode of the eighth thin film transistor5578 is connected to the gate electrode of the second thin filmtransistor 5572. A gate electrode of the eighth thin film transistor5578 is connected to the first wiring 5501.

Note that the point at which the gate electrode of the first thin filmtransistor 5571, the gate electrode of the fourth thin film transistor5574, the second electrode of the fifth thin film transistor 5575, thesecond electrode of the sixth thin film transistor 5576, and the secondelectrode of the seventh thin film transistor 5577 are connected isreferred to as a node 5543. The point at which the gate electrode of thesecond thin film transistor 5572, the second electrode of the third thinfilm transistor 5573, the second electrode of the fourth thin filmtransistor 5574, the gate electrode of the sixth thin film transistor5576, and the second electrode of the eighth thin film transistor 5578are connected is referred to as a node 5544.

The first wiring 5501, the second wiring 5502, the third wiring 5503,and the fourth wiring 5504 may be referred to as a first signal line, asecond signal line, a third signal line, and a fourth signal line,respectively. The fifth wiring 5505 and the sixth wiring 5506 may bereferred to as a first power supply line and a second power supply line,respectively.

In addition, the signal line driver circuit and the scan line drivercircuit can be formed using only the n-channel TFTs described inEmbodiment 4. The n-channel TFT described in Embodiment 4 has a highmobility, and thus a driving frequency of a driver circuit can beincreased. Further, parasitic capacitance is reduced by the source ordrain region which is an In—Ga—Zn—O-based non-single-crystal film; thus,the n-channel TFT described in Embodiment 4 has high frequencycharacteristics (referred to as f characteristics). For example, a scanline driver circuit using the n-channel TFT described in Embodiment 4can operate at high speed, and thus a frame frequency can be increasedand insertion of black images can be realized.

In addition, when the channel width of the transistor in the scan linedriver circuit is increased or a plurality of scan line driver circuitsare provided, for example, higher frame frequency can be realized. Whena plurality of scan line driver circuits is provided, a scan line drivercircuit for driving even-numbered scan lines is provided on one side anda scan line driver circuit for driving odd-numbered scan lines isprovided on the opposite side; thus, increase in frame frequency can berealized. Furthermore, the use of the plurality of scan line drivercircuits for output of signals to the same scan line is advantageous inincreasing the size of a display device.

In the case of manufacturing an active matrix light-emitting displaydevice, which is an example of a semiconductor device of the presentinvention, a plurality of scan line driver circuits are preferablyarranged because a plurality of thin film transistors are arranged in atleast one pixel. An example of a block diagram of an active matrixlight-emitting display device is illustrated in FIG. 14B.

The light-emitting display device illustrated in FIG. 14B includes, overa substrate 5400, a pixel portion 5401 including a plurality of pixelseach provided with a display element, a first scan line driver circuit5402 and a second scan line driver circuit 5404 that select a pixel, anda signal line driver circuit 5403 that controls a video signal input tothe selected pixel.

In the case of inputting a digital video signal to the pixel of thelight-emitting display device of FIG. 14B, the pixel is put in alight-emitting state or non-light-emitting state by switching on/off ofthe transistor. Thus, grayscale can be displayed using an area ratiograyscale method or a time ratio grayscale method. An area ratiograyscale method refers to a driving method by which one pixel isdivided into a plurality of subpixels and the respective subpixels aredriven separately based on video signals so that grayscale is displayed.Further, a time ratio grayscale method refers to a driving method bywhich a period during which a pixel is in a light-emitting state iscontrolled so that grayscale is displayed.

Since the response time of light-emitting elements is shorter than thatof liquid crystal elements or the like, the light-emitting elements aresuitable for a time ratio grayscale method. Specifically, in the case ofdisplaying by a time grayscale method, one frame period is divided intoa plurality of subframe periods. Then, in accordance with video signals,the light-emitting element in the pixel is put in a light-emitting stateor a non-light-emitting state in each subframe period. By dividing aframe into a plurality of subframes, the total length of time in whichpixels actually emit light in one frame period can be controlled withvideo signals to display grayscales.

Note that in the light-emitting display device of FIG. 14B, in the casewhere one pixel includes two switching TFTs, a signal which is input toa first scan line which is a gate wiring of one of the switching TFTs isgenerated in the first scan line driver circuit 5402 and a signal whichis input to a second scan line which is a gate wiring of the otherswitching TFT is generated in the second scan line driver circuit 5404.However, both of the signals which are input to the first scan line andthe second scan line may be generated in one scan line driver circuit.In addition, for example, there is a possibility that a plurality ofscan lines used for controlling the operation of the switching elementsbe provided in each pixel depending on the number of switching TFTsincluded in one pixel. In this case, the signals which are input to thescan lines may all be generated in one scan line driver circuit or maybe generated in a plurality of scan line driver circuits.

Also in the light-emitting display device, part of the driver circuitwhich can be formed using the n-channel TFTs can be provided over onesubstrate together with the thin film transistors of the pixel portion.Moreover, the signal line driver circuit and the scan line drivercircuit can be manufactured using only the n-channel TFTs described inEmbodiment 4 or 5.

The above driver circuit may be used for not only a liquid crystaldisplay device or a light-emitting display device but also an electronicpaper in which electronic ink is driven by utilizing an elementelectrically connected to a switching element. The electronic paper isalso called an electrophoretic display device (electrophoretic display)and has advantages in that it has the same level of readability asregular paper, it has less power consumption than other display devices,and it can be made thin and lightweight.

There are a variety of modes of electrophoretic displays. Theelectrophoretic display is a device in which a plurality ofmicrocapsules each including first particles having positive charge andsecond particles having negative charge are dispersed in a solvent or asolute, and an electrical field is applied to the microcapsules so thatthe particles in the microcapsules move in opposite directions from eachother, and only a color of the particles gathered on one side isdisplayed. Note that the first particles or the second particles includea colorant, and does not move when there is not electric field. Also, acolor of the first particles is different from a color of the secondparticles (the particles may also be colorless).

Thus, the electrophoretic display utilizes a so-called dielectrophoreticeffect, in which a substance with high dielectric constant moves to aregion with high electric field. The electrophoretic display does notrequire a polarizing plate and a counter substrate, which are necessaryfor a liquid crystal display device, so that the thickness and weightthereof are about half.

In electronic ink the microcapsules are dispersed in a solvent, and thiselectronic ink can be printed on a surface of glass, plastic, fabric,paper, or the like. Color display is also possible with the use of acolor filter or particles including a coloring matter.

In addition, an active matrix display device can be completed byproviding as appropriate, a plurality of the microcapsules over anactive matrix substrate so as to be interposed between two electrodes,and can perform display by application of electric field to themicrocapsules. For example, the active matrix substrate obtained usingthe thin film transistors described in Embodiment 4 or 5 can be used.

Note that the first particles and the second particles in themicrocapsules may be formed from one of a conductive material, aninsulating material, a semiconductor material, a magnetic material, aliquid crystal material, a ferroelectric material, an electroluminescentmaterial, an electrochromic material, and a magnetophoretic material ora composite material thereof.

Through the above process, a highly reliable display device as asemiconductor device can be manufactured.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 7

A thin film transistor of an embodiment of the present invention ismanufactured, and the thin film transistor can be used for a pixelportion and further for a driver circuit, so that a semiconductor devicehaving a display function (also called a display device) can bemanufactured. Moreover, the thin film transistor of an embodiment of thepresent invention can be used for part of a driver circuit or an entiredriver circuit formed over one substrate together with a pixel portion,so that a system-on-panel can be formed.

The display device includes a display element. As the display element, aliquid crystal element (also referred to as a liquid crystal displayelement) or a light-emitting element (also referred to as alight-emitting display element) can be used. A light-emitting elementincludes, in its category, an element whose luminance is controlled bycurrent or voltage, and specifically includes an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Further, a display medium whose contrast is changed by an electriceffect, such as electronic ink, can be used.

In addition, the display device includes a panel in which a displayelement is sealed, and a module in which an IC and the like including acontroller are mounted on the panel. An embodiment of the presentinvention further relates to one mode of an element substrate before thedisplay element is completed in a process for manufacturing the displaydevice, and the element substrate is provided with a plurality of pixelseach having a means for supplying current to the display element.Specifically, the element substrate may be in a state after only a pixelelectrode of the display element is formed, a state after a conductivefilm to be a pixel electrode is formed but before the conductive film isetched to be the pixel electrode, or any other states.

A display device in this specification refers to an image displaydevice, a display device, or a light source (including a lightingdevice). Further, the display device also includes any of the followingmodules in its category: a module to which a connector such as aflexible printed circuit (FPC), a tape automated bonding (TAB) tape, ora tape carrier package (TCP) is attached; a module having a TAB tape ora TCP at the end of which a printed wiring board is provided; and amodule in which an integrated circuit (IC) is directly mounted on adisplay element by a chip-on-glass (COG) method.

The appearance and a cross section of a liquid crystal display panelwhich is one mode of a semiconductor device according to an embodimentof the present invention will be described in this embodiment withreference to FIGS. 22A1, 22A2, and 22B. FIGS. 22A1 and 22A2 are topviews of panels in which thin film transistors 4010 and 4011 and aliquid crystal element 4013 which are formed over a first substrate 4001are sealed with a sealant 4005 between the first substrate 4001 and asecond substrate 4006. The thin film transistors 4010 and 4011 which arethin film transistors described in Embodiment 4 include anIn—Ga—Zn—O-based non-single-crystal film as a semiconductor layer andhave high reliability. FIG. 22B is a cross-sectional view taken alongM-N of FIGS. 22A1 and 22A2.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line driver circuit 4004 which are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scan line driver circuit 4004. Thus, the pixelportion 4002 and the scan line driver circuit 4004 as well as a liquidcrystal layer 4008 are sealed with the sealant 4005 between the firstsubstrate 4001 and the second substrate 4006. A signal line drivercircuit 4003 that is formed using a single crystal semiconductor film ora polycrystalline semiconductor film over a substrate separatelyprepared is mounted in a region that is different from the regionsurrounded by the sealant 4005 over the first substrate 4001.

Note that there is no particular limitation on a connection method ofthe driver circuit which is separately formed, and a COG method, a wirebonding method, a TAB method, or the like can be used. FIG. 22A1illustrates an example in which the signal line driver circuit 4003 ismounted by a COG method and FIG. 22A2 illustrates an example in whichthe signal line driver circuit 4003 is mounted by a TAB method.

Each of the pixel portion 4002 and the scan line driver circuit 4004which are provided over the first substrate 4001 includes a plurality ofthin film transistors. FIG. 22B illustrates the thin film transistor4010 included in the pixel portion 4002 and the thin film transistor4011 included in the scan line driver circuit 4004. Insulating layers4020 and 4021 are provided over the thin film transistors 4010 and 4011.

As the thin film transistors 4010 and 4011, the thin film transistorincluding an In—Ga—Zn—O-based non-single-crystal film as a semiconductorlayer and having high reliability which is described in Embodiment 4 canbe employed. Alternatively, the thin film transistor described inEmbodiment 5 may be employed. In this embodiment, the thin filmtransistors 4010 and 4011 are n-channel thin film transistors.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to the thin film transistor 4010. A counterelectrode layer 4031 of the liquid crystal element 4013 is formed on thesecond substrate 4006. A portion where the pixel electrode layer 4030,the counter electrode layer 4031, and the liquid crystal layer 4008overlap with each other corresponds to the liquid crystal element 4013.Note that the pixel electrode layer 4030 and the counter electrode layer4031 are provided with an insulating layer 4032 and an insulating layer4033 serving as alignment films, respectively, and hold the liquidcrystal layer 4008 with the insulating layers 4032 and 4033 interposedtherebetween.

Note that the first substrate 4001 and the second substrate 4006 can beformed from glass, metal (typically, stainless steel), ceramic, orplastic. As plastic, a fiberglass-reinforced plastics (FRP) plate, apolyvinyl fluoride (PVF) film, a polyester film, or an acrylic resinfilm can be used. Alternatively, a sheet with a structure in which analuminum foil is sandwiched between PVF films or polyester films can beused.

A columnar spacer denoted by reference numeral 4035 is obtained byselective etching of an insulating film and is provided in order tocontrol the distance (a cell gap) between the pixel electrode layer 4030and the counter electrode layer 4031. Note that a spherical spacer maybe used. The counter electrode layer 4031 is electrically connected to acommon potential line provided over the same substrate as the thin filmtransistor 4010. With the use of the common connection portion, thecounter electrode layer 4031 can be electrically connected to the commonpotential line through conductive particles provided between the pair ofsubstrates. Note that the conductive particles are contained in thesealant 4005.

Alternatively, a blue phase liquid crystal for which an alignment filmis unnecessary may be used. A blue phase is a type of liquid crystalphase, which appears just before a cholesteric liquid crystal changesinto an isotropic phase when the temperature of the cholesteric liquidcrystal is increased. A blue phase appears only within a narrowtemperature range; therefore, the liquid crystal layer 4008 is formedusing a liquid crystal composition containing a chiral agent at 5 wt. %or more in order to expand the temperature range. The liquid crystalcomposition including a blue phase liquid crystal and a chiral agent hasa short response time of 10 μs to 100 μs, and is optically isotropic;therefore, alignment treatment is not necessary and viewing angledependence is small.

Note that this embodiment describes an example of a transmissive liquidcrystal display device; however, an embodiment of the present inventioncan be applied to a reflective liquid crystal display device or atransflective liquid crystal display device.

Although a liquid crystal display device of this embodiment has apolarizing plate provided outer than the substrate (the viewer side) anda coloring layer and an electrode layer of a display element providedinner than the substrate, which are arranged in that order, thepolarizing plate may be inner than the substrate. The stacked structureof the polarizing plate and the coloring layer is not limited to thatshown in this embodiment and may be set as appropriate in accordancewith the materials of the polarizing plate and the coloring layer andthe condition of the manufacturing process. Further, a light-blockingfilm serving as a black matrix may be provided.

In this embodiment, in order to reduce the unevenness of the surface ofthe thin film transistors and to improve the reliability of the thinfilm transistors, the thin film transistors which are obtained inEmbodiment 4 are covered with protective films or insulating layers (theinsulating layers 4020 and 4021) serving as planarizing insulatingfilms. Note that the protective film is provided to prevent entry ofimpurities floating in air, such as an organic substance, a metalsubstance, or moisture, and is preferably a dense film. The protectivefilm may be formed by a sputtering method using a single layer or astack of layers of a silicon oxide film, a silicon nitride film, asilicon oxynitride film, a silicon nitride oxide film, an aluminum oxidefilm, an aluminum nitride film, an aluminum oxynitride film, or analuminum nitride oxide film. Although the protective film is formed by asputtering method in this embodiment, the method is not limited to aparticular method and may be selected from a variety of methods.

Here, the insulating layer 4020 is formed to have a stacked structure asthe protective film. Here, a silicon oxide film is formed by asputtering method as a first layer of the insulating layer 4020. The useof a silicon oxide film for the protective film provides an advantageouseffect of preventing hillock of an aluminum film used for a sourceelectrode layer and a drain electrode layer.

Moreover, an insulating layer is formed as a second layer of theprotective film. Here, a silicon nitride film is formed by a sputteringmethod as a second layer of the insulating layer 4020. When a siliconnitride film is used for the protective film, it is possible to preventmovable ions such as sodium from entering a semiconductor region tochange the electrical characteristics of the TFT.

Further, after the protective film is formed, the semiconductor layermay be annealed (at 300° C. to 400° C.).

Further, the insulating layer 4021 is formed as the planarizinginsulating film. The insulating layer 4021 can be formed from an organicmaterial having heat resistance, such as polyimide, acrylic,benzocyclobutene, polyamide, or epoxy. As an alternative to such organicmaterials, it is possible to use a low-dielectric constant material (alow-k material), a siloxane-based resin, phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), or the like. Note that the insulatinglayer 4021 may be formed by stacking a plurality of insulating filmsformed from these materials.

Note that a siloxane-based resin is a resin formed from a siloxane-basedmaterial as a starting material and having the bond of Si—O—Si. Thesiloxane-based resin may include an organic group (such as an alkylgroup and an aryl group) or a fluoro group as a substituent. The organicgroup may include a fluoro group.

The method for the formation of the insulating layer 4021 is not limitedto a particular method and the following method can be used depending onthe material of the insulating layer 4021: a sputtering method, an SOGmethod, spin coating, dip coating, spray coating, a droplet dischargemethod (e.g., an inkjet method, screen printing, or offset printing), adoctor knife, a roll coater, a curtain coater, a knife coater, or thelike. In the case of forming the insulating layer 4021 with the use of amaterial solution, annealing (at 300° C. to 400° C.) may be performed ona semiconductor layer at the same time as a baking step. When the bakingstep of the insulating layer 4021 and the annealing of the semiconductorlayer are combined, a semiconductor device can be manufacturedefficiently.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe formed from a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter referred to asITO), indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

A conductive composition including a conductive high molecule (alsoreferred to as a conductive polymer) can be used for the pixel electrodelayer 4030 and the counter electrode layer 4031. The pixel electrodeformed of the conductive composition has preferably a sheet resistanceof 10000 Ω/square or less and a transmittance of 70% or more at awavelength of 550 nm. Further, the resistivity of the conductive highmolecule included in the conductive composition is preferably 0.1 Ω·cmor less.

As the conductive high molecule, a so-called π-electron conjugatedconductive polymer can be used. As examples thereof, polyaniline or aderivative thereof, polypyrrole or a derivative thereof, polythiopheneor a derivative thereof, a copolymer of two or more of them can begiven.

Further, a variety of signals and potentials are supplied from an FPC4018 to the signal line driver circuit 4003 which is formed separately,the scan line driver circuit 4004, and the pixel portion 4002.

In this embodiment, a connecting terminal electrode 4015 is formed usingthe same conductive film as the pixel electrode layer 4030 included inthe liquid crystal element 4013, and a terminal electrode 4016 is formedusing the same conductive film as the source and drain electrode layersof the thin film transistors 4010 and 4011.

The connecting terminal electrode 4015 is electrically connected to aterminal of the FPC 4018 through an anisotropic conductive film 4019.

Although FIGS. 22A1, 22A2, and 22B show an example in which the signalline driver circuit 4003 is formed separately and mounted on the firstsubstrate 4001, this embodiment is not limited to this structure. Thescan line driver circuit may be separately formed and then mounted, oronly part of the signal line driver circuit or part of the scan linedriver circuit may be separately formed and then mounted.

FIG. 23 illustrates an example in which a liquid crystal display moduleis formed as a semiconductor device using a TFT substrate 2600manufactured according to an embodiment of the present invention.

FIG. 23 illustrates an example of a liquid crystal display module, inwhich the TFT substrate 2600 and a counter substrate 2601 are fixed toeach other with a sealant 2602, and a pixel portion 2603 including a TFTand the like, a display element 2604 including a liquid crystal layer,and a coloring layer 2605 are provided between the substrates to form adisplay region. The coloring layer 2605 is necessary to perform colordisplay. In the case of the RGB system, coloring layers corresponding tocolors of red, green, and blue are provided for pixels. Polarizingplates 2606 and 2607 and a diffuser plate 2613 are provided outside theTFT substrate 2600 and the counter substrate 2601. A light sourceincludes a cold cathode tube 2610 and a reflective plate 2611, and acircuit board 2612 is connected to a wiring circuit portion 2608 of theTFT substrate 2600 through a flexible wiring board 2609 and includes anexternal circuit such as a control circuit and a power source circuit.The polarizing plate and the liquid crystal layer may be stacked with aretardation plate interposed therebetween.

For the liquid crystal display module, a TN (twisted nematic) mode, anIPS (in-plane-switching) mode, an FFS (fringe field switching) mode, anMVA (multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASM (axially symmetric aligned micro-cell) mode, anOCB (optically compensated birefringence) mode, an FLC (ferroelectricliquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode,or the like can be employed.

Through the above process, a highly reliable liquid crystal displaypanel as a semiconductor device can be manufactured.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 8

In this embodiment, an example of an electronic paper will be describedas a semiconductor device of an embodiment of the present invention.

FIG. 13 illustrates an active matrix electronic paper as an example of asemiconductor device to which an embodiment of the present invention isapplied. A thin film transistor 581 used for the semiconductor devicecan be manufactured in a manner similar to that of the thin filmtransistor described in Embodiment 4 and is a highly reliable thin filmtransistor including an In—Ga—Zn—O-based non-single-crystal film as asemiconductor layer. The thin film transistor described in Embodiment 5can also be used as the thin film transistor 581 in this embodiment.

The electronic paper of FIG. 13 is an example of a display device inwhich a twisting ball display system is employed. The twisting balldisplay system refers to a method in which spherical particles eachcolored in black and white are arranged between a first electrode layerand a second electrode layer which are electrode layers used for adisplay element, and a potential difference is generated between thefirst electrode layer and the second electrode layer to controlorientation of the spherical particles, so that display is performed.

The thin film transistor 581 is a thin film transistor with a bottomgate structure, and a source or drain electrode layer thereof is incontact with a first electrode layer 587 through an opening formed in aninsulating layer 585, whereby the thin film transistor 581 iselectrically connected to the first electrode layer 587. Between thefirst electrode layer 587 and a second electrode layer 588, sphericalparticles 589 each having a black region 590 a, a white region 590 b,and a cavity 594 around the regions which is filled with liquid areprovided. A space around the spherical particles 589 is filled with afiller 595 such as a resin (see FIG. 13). In this embodiment, the firstelectrode layer 587 corresponds to a pixel electrode, and the secondelectrode layer 588 corresponds to a common electrode. The secondelectrode layer 588 is electrically connected to a common potential lineprovided over the same substrate as the thin film transistor 581. Withthe use of a common connection portion which is described in any ofEmbodiments 1 to 3, the second electrode layer 588 can be electricallyconnected to the common potential line through conductive particlesprovided between a pair of substrates.

Instead of the twisting ball, an electrophoretic element can also beused. A microcapsule having a diameter of about 10 to 200 μm in whichtransparent liquid, positively-charged white microparticles, andnegatively-charged black microparticles are encapsulated, is used. Inthe microcapsule which is provided between the first electrode layer andthe second electrode layer, when an electric field is applied betweenthe first electrode layer and the second electrode layer, the whitemicroparticles and the black microparticles move to opposite sides fromeach other, so that white or black can be displayed. A display elementusing this principle is an electrophoretic display element and isgenerally called an electronic paper. The electrophoretic displayelement has higher reflectance than a liquid crystal display element,and thus, an auxiliary light is unnecessary, power consumption is low,and a display portion can be recognized in a dim place. In addition,even when power is not supplied to the display portion, an image whichhas been displayed once can be maintained. Accordingly, a displayedimage can be stored even if a semiconductor device having a displayfunction (which may be referred to simply as a display device or asemiconductor device provided with a display device) is distanced froman electric wave source.

Through this process, a highly reliable electronic paper as asemiconductor device can be manufactured.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 9

This embodiment describes an example of a light-emitting display deviceas a semiconductor device according to an embodiment of the presentinvention. As an example of a display element of the display device,here, a light-emitting element utilizing electroluminescence isdescribed. Light-emitting elements utilizing electroluminescence areclassified according to whether a light emitting material is an organiccompound or an inorganic compound. In general, the former is referred toas an organic EL element and the latter is referred to as an inorganicEL element.

In an organic EL element, by application of a voltage to alight-emitting element, electrons and holes are separately injected froma pair of electrodes into a layer containing a light-emitting organiccompound, and thus current flows. Then, those carriers (i.e., electronsand holes) are recombined, and thus, the light-emitting organic compoundis excited. When the light-emitting organic compound returns to a groundstate from the excited state, light is emitted. Owing to such amechanism, this light emitting element is referred to as acurrent-excitation light emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion type inorganic EL element and a thin-filmtype inorganic EL element. A dispersion type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film type inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that an organic ELelement is used as a light-emitting element in this example.

FIG. 20 illustrates an example of a pixel structure to which digitaltime grayscale driving can be applied, as an example of a semiconductordevice to which an embodiment of the present invention is applied.

A structure and operation of a pixel to which digital time grayscaledriving can be applied are described. In this example, one pixelincludes two n-channel transistors in each of which an oxidesemiconductor layer (an In—Ga—Zn—O-based non-single-crystal film) in achannel formation region.

A pixel 6400 includes a switching transistor 6401, a driver transistor6402, a light-emitting element 6404, and a capacitor 6403. A gate of theswitching transistor 6401 is connected to a scan line 6406, a firstelectrode (one of a source electrode and a drain electrode) of theswitching transistor 6401 is connected to a signal line 6405, and asecond electrode (the other of the source electrode and the drainelectrode) of the switching transistor 6401 is connected to a gate ofthe driver transistor 6402. The gate of the driver transistor 6402 isconnected to a power supply line 6407 through the capacitor 6403, afirst electrode of the driver transistor 6402 is connected to the powersupply line 6407, and a second electrode of the driver transistor 6402is connected to a first electrode (pixel electrode) of thelight-emitting element 6404. A second electrode of the light-emittingelement 6404 corresponds to a common electrode 6408. The commonelectrode 6408 is electrically connected to a common potential lineprovided over the same substrate, and the structure illustrated in FIG.1A, FIG. 2A, or FIG. 3A may be obtained using the connection portion asa common connection portion.

The second electrode (common electrode 6408) of the light-emittingelement 6404 is set to a low power supply potential. The low powersupply potential is a potential smaller than a high power supplypotential when the high power supply potential set to the power supplyline 6407 is a reference. As the low power supply potential, GND, 0 V,or the like may be employed, for example. A potential difference betweenthe high power supply potential and the low power supply potential isapplied to the light-emitting element 6404 and current is supplied tothe light-emitting element 6404, so that the light-emitting element 6404emits light. In order to make the light-emitting element 6404 emitlight, potentials are set so that the potential difference between thehigh power supply potential and the low power supply potential isgreater than or equal to the forward threshold voltage of thelight-emitting element 6404.

Gate capacitance of the driver transistor 6402 may be used as asubstitute for the capacitor 6403, so that the capacitor 6403 can beomitted. The gate capacitance of the driver transistor 6402 may beformed between the channel region and the gate electrode.

In the case of a voltage-input voltage driving method, a video signal isinput to the gate of the driver transistor 6402 so that the drivertransistor 6402 is either substantially turned on or substantiallyturned off. That is, the driver transistor 6402 operates in a linearregion. Since the driver transistor 6402 operates in a linear region, avoltage higher than the voltage of the power supply line 6407 is appliedto the gate of the driver transistor 6402. Note that a voltage which ishigher than or equal to the sum of the voltage of the power supply lineand the Vth of the driver transistor 6402 is applied to the signal line6405.

In the case of performing analog grayscale driving instead of digitaltime grayscale driving, the same pixel structure as that in FIG. 20 canbe used by changing signal input.

In the case of performing analog grayscale driving, a voltage higherthan or equal to the sum of the forward voltage of the light-emittingelement 6404 and the Vth of the driver transistor 6402 is applied to thegate of the driver transistor 6402. The forward voltage of thelight-emitting element 6404 indicates a voltage at which a desiredluminance is obtained, and includes at least a forward thresholdvoltage. The video signal by which the driver transistor 6402 operatesin a saturation region is input, so that current can be supplied to thelight-emitting element 6404. In order for the driver transistor 6402 tooperate in a saturation region, the potential of the power supply line6407 is set higher than the gate potential of the driver transistor6402. When an analog video signal is used, it is possible to feedcurrent to the light-emitting element 6404 in accordance with the videosignal and perform analog grayscale driving.

Note that the pixel structure shown in FIG. 20 is not limited thereto.For example, a switch, a resistor, a capacitor, a transistor, a logiccircuit, or the like may be added to the pixel shown in FIG. 20.

Next, structures of a light-emitting element are described withreference to FIGS. 21A to 21C. A cross-sectional structure of a pixel isdescribed here by taking an n-channel driver TFT as an example. TFTs7001, 7011, and 7021 serving as driver TFTs used for a semiconductordevice, which are illustrated in FIGS. 21A to 21C, can be formed by amethod similar to the method for forming the thin film transistordescribed in Embodiment 4. The TFTs 7001, 7011, and 7021 have highreliability and each include an In—Ga—Zn—O-based non-single-crystal filmas a semiconductor layer. The thin film transistor described inEmbodiment 5 can also be used as the TFTs 7001, 7011, and 7021.

In addition, in order to extract light emitted from the light-emittingelement, at least one of an anode and a cathode is required to transmitlight. A thin film transistor and a light-emitting element are formedover a substrate. A light-emitting element can have a top-emissionstructure in which light is extracted through the surface opposite tothe substrate; a bottom-emission structure in which light is extractedthrough the surface on the substrate side; or a dual-emission structurein which light is extracted through the surface opposite to thesubstrate and the surface on the substrate side. The pixel structureaccording to an embodiment of the present invention can be applied to alight-emitting element having any of these emission structures.

A light-emitting element with a top-emission structure is described withreference to FIG. 21A.

FIG. 21A is a cross-sectional view of a pixel in a case where the TFT7001 serving as a driver TFT is an n-channel TFT and light generated ina light-emitting element 7002 is emitted to an anode 7005 side. In FIG.21A, a cathode 7003 of the light-emitting element 7002 is electricallyconnected to the TFT 7001 serving as a driver TFT, and a light-emittinglayer 7004 and the anode 7005 are stacked in this order over the cathode7003. The cathode 7003 can be formed using any of conductive materialswhich have a low work function and reflect light. For example, Ca, Al,CaF, MgAg, AlLi, or the like is preferably used. The light-emittinglayer 7004 may be formed using a single layer or by stacking a pluralityof layers. When the light-emitting layer 7004 is formed using aplurality of layers, the light-emitting layer 7004 is formed by stackingan electron-injecting layer, an electron-transporting layer, alight-emitting layer, a hole-transporting layer, and a hole-injectinglayer in this order over the cathode 7003. It is not necessary to formall of these layers. The anode 7005 is formed using a light-transmittingconductive film formed from a light-transmitting conductive materialsuch as indium oxide including tungsten oxide, indium zinc oxideincluding tungsten oxide, indium oxide including titanium oxide, indiumtin oxide including titanium oxide, indium tin oxide (hereinafter,referred to as ITO), indium zinc oxide, or indium tin oxide to whichsilicon oxide is added.

The light-emitting element 7002 corresponds to a region where thecathode 7003 and the anode 7005 sandwich the light-emitting layer 7004.In the case of the pixel illustrated in FIG. 21A, light is emitted fromthe light-emitting element 7002 to the anode 7005 side as indicated byan arrow.

Next, a light-emitting element having a bottom-emission structure isdescribed with reference to FIG. 21B. FIG. 21B is a cross-sectional viewof a pixel in the case where the driver TFT 7011 is an n-channel TFT,and light is emitted from a light-emitting element 7012 to a cathode7013 side. In FIG. 21B, the cathode 7013 of the light-emitting element7012 is formed over a conductive film 7017 having a light-transmittingproperty which is electrically connected to the driver TFT 7011, and alight-emitting layer 7014 and an anode 7015 are stacked in this orderover the cathode 7013. A light-blocking film 7016 for reflecting orblocking light may be formed so as to cover the anode 7015 when theanode 7015 has a light-transmitting property. For the cathode 7013, anyof conductive materials which have a low work function can be used as inthe case of FIG. 21A. Note that the cathode 7013 is formed to have athickness with which the cathode 7013 can transmit light (preferably,approximately from 5 nm to 30 nm). For example, an aluminum film with athickness of 20 nm can be used as the cathode 7013. The light-emittinglayer 7014 may be formed of a single layer or by stacking a plurality oflayers as in the case of FIG. 21A. The anode 7015 is not required totransmit light, but can be formed using a light-transmitting conductivematerial as in the case of FIG. 21A. For the light-blocking film 7016,metal or the like that reflects light can be used; however, it is notlimited to a metal film. For example, a resin or the like to which blackpigment is added can be used.

The light-emitting element 7012 corresponds to a region where thecathode 7013 and the anode 7015 sandwich the light-emitting layer 7014.In the case of the pixel illustrated in FIG. 21B, light is emitted fromthe light-emitting element 7012 to the cathode 7013 side as indicated byan arrow.

Next, a light-emitting element having a dual-emission structure isdescribed with reference to FIG. 21C. In FIG. 21C, a cathode 7023 of alight-emitting element 7022 is formed over a conductive film 7027 havinga light-transmitting property which is electrically connected to thedriver TFT 7021, and a light-emitting layer 7024 and an anode 7025 arestacked in this order over the cathode 7023. As in the case of FIG. 21A,the cathode 7023 can be formed of any of conductive materials which havea low work function. Note that the cathode 7023 is formed to have athickness with which the cathode 7023 can transmit light. For example,an Al film having a thickness of 20 nm can be used as the cathode 7023.The light-emitting layer 7024 may be formed using a single layer or bystacking a plurality of layers as in the case of FIG. 21A. In a mannersimilar to FIG. 21A, the anode 7025 can be formed using alight-transmitting conductive material.

The light-emitting element 7022 corresponds to a region where thecathode 7023, the light-emitting layer 7024, and the anode 7025 overlapwith each other. In the pixel illustrated in FIG. 21C, light is emittedfrom the light-emitting element 7022 to both the anode 7025 side and thecathode 7023 side as indicated by arrows.

Although an organic EL element is described here as a light-emittingelement, an inorganic EL element can be alternatively provided as alight-emitting element.

Note that this embodiment describes the example in which a thin filmtransistor (driver TFT) which controls the driving of a light-emittingelement is electrically connected to the light-emitting element, but astructure may be employed in which a current control TFT is connectedbetween the driver TFT and the light-emitting element.

The semiconductor device described in this embodiment is not limited tothe structures illustrated in FIGS. 21A to 21C, and can be modified invarious ways based on the spirit of techniques according to the presentinvention.

Next, the appearance and cross section of a light-emitting display panel(also referred to as a light-emitting panel) which is one mode of asemiconductor device according to the present invention will bedescribed with reference to FIGS. 24A and 24B. FIG. 24A is a top view ofa panel in which a light-emitting element and a thin film transistorover a first substrate are sealed with a sealant between the firstsubstrate and a second substrate. FIG. 24B is a cross-sectional viewalong H-I of FIG. 24A.

A sealant 4505 is provided so as to surround a pixel portion 4502,signal line driver circuits 4503 a and 4503 b, and scan line drivercircuits 4504 a and 4504 b, which are provided over a first substrate4501. In addition, a second substrate 4506 is formed over the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b. Accordingly, the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b are sealed, together with afiller 4507, with the first substrate 4501, the sealant 4505, and thesecond substrate 4506. In this manner, it is preferable that thelight-emitting display panel be packaged (sealed) with a protective film(such as an attachment film or an ultraviolet curable resin film) or acover material with high air-tightness and little degasification so asnot to be exposed to external air.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b formed over thefirst substrate 4501 each include a plurality of thin film transistors.A thin film transistor 4510 included in the pixel portion 4502 and athin film transistor 4509 included in the signal line driver circuit4503 a are illustrated as an example in FIG. 24B.

As the thin film transistors 4509 and 4510, a thin film transistordescribed in Embodiment 4 which has high reliability and includes anIn—Ga—Zn—O-based non-single-crystal film as a semiconductor layer can beused. Further, the thin film transistor described in Embodiment 5 can beused as the thin film transistors 4509 and 4510. In this embodiment, thethin film transistors 4509 and 4510 are n-channel thin film transistors.

Moreover, reference numeral 4511 denotes a light-emitting element. Afirst electrode layer 4517 which is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a source ordrain electrode layer of the thin film transistor 4510. Note thatalthough the light-emitting element 4511 has a stacked structure of thefirst electrode layer 4517, an electroluminescent layer 4512, and asecond electrode layer 4513, the structure of the light-emitting element4511 is not limited thereto. The structure of the light-emitting element4511 can be changed as appropriate depending on a direction in whichlight is extracted from the light-emitting element 4511, or the like.

A partition wall 4520 is formed using an organic resin film, aninorganic insulating film, or organic polysiloxane. It is particularlypreferable that the partition wall 4520 be formed using a photosensitivematerial to have an opening portion on the first electrode layer 4517 sothat a sidewall of the opening portion is formed as an inclined surfacewith a continuous curvature.

The electroluminescent layer 4512 may be formed using a single layer ora plurality of layers stacked.

In order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide,or the like into the light-emitting element 4511, a protective film maybe formed over the second electrode layer 4513 and the partition wall4520. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed.

In addition, a variety of signals and potentials are supplied from FPCs4518 a and 4518 b to the signal line driver circuits 4503 a and 4503 b,the scan line driver circuits 4504 a and 4504 b, or the pixel portion4502.

In this embodiment, a connecting terminal electrode 4515 is formed usingthe same conductive film as the first electrode layer 4517 included inthe light-emitting element 4511. A terminal electrode 4516 is formedusing the same conductive film as the source and drain electrode layersincluded in the thin film transistors 4509 and 4510.

The connecting terminal electrode 4515 is electrically connected to aterminal included in the FPC 4518 a through an anisotropic conductivefilm 4519.

The second substrate located in the direction in which light isextracted from the light-emitting element 4511 needs to have alight-transmitting property. In that case, a light-transmitting materialsuch as a glass plate, a plastic plate, a polyester film, or an acrylicfilm is used.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used as well as inert gas such as nitrogen or argon. Forexample, polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, asilicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA)can be used. In this embodiment, nitrogen is used for the filler 4507.

In addition, if needed, an optical film such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),and a color filter may be provided as appropriate on an emission surfaceof the light-emitting element. Further, the polarizing plate or thecircularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment can be performed by whichreflected light is diffused by depressions and projections of thesurface and glare can be reduced.

As the signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b, driver circuits formed by using asingle crystal semiconductor film or a polycrystalline semiconductorfilm over a substrate separately prepared may be mounted. In addition,only the signal line driver circuits or only part thereof, or only thescan line driver circuits or only part thereof may be separately formedand then mounted. This embodiment is not limited to the structure shownin FIGS. 24A and 24B.

Through this process, a highly reliable light-emitting device (displaypanel) as a semiconductor device can be manufactured.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 10

A semiconductor device according to an embodiment of the presentinvention can be applied as an electronic paper. An electronic paper canbe used for electronic appliances of a variety of fields for displayinginformation. For example, an electronic paper can be used for electronicbook reader (an e-book reader), posters, advertisement in vehicles suchas trains, displays of various cards such as credit cards, and the like.Examples of such electronic appliances are illustrated in FIGS. 25A and25B and FIG. 26.

FIG. 25A illustrates a poster 2631 formed using an electronic paper. Ifthe advertising medium is printed paper, the advertisement is replacedby hands; however, when an electronic paper to which an embodiment ofthe present invention is applied is used, the advertisement display canbe changed in a short time. Moreover, a stable image can be obtainedwithout defects. Further, the poster may send and receive informationwirelessly.

FIG. 25B illustrates an advertisement 2632 in a vehicle such as a train.If the advertising medium is printed paper, the advertisement isreplaced by hands; however, when an electronic paper to which anembodiment of the present invention is applied is used, theadvertisement display can be changed in a short time without muchmanpower. Moreover, a stable image can be obtained without defects.Further, the advertisement in vehicles may send and receive informationwirelessly.

FIG. 26 illustrates an example of an electronic book reader 2700. Forexample, the electronic book reader 2700 includes two housings 2701 and2703. The housings 2701 and 2703 are bonded with a hinge 2711 so thatthe electronic book reader 2700 can be opened and closed along the hinge2711. With such a structure, the electronic book reader 2700 can behandled like a paper book.

A display portion 2705 is incorporated in the housing 2701 and a displayportion 2707 is incorporated in the housing 2703. The display portion2705 and the display portion 2707 may display one image, or may displaydifferent images. In the structure where different images are displayedon the display portion 2705 and the display portion 2707, for example,the right display portion (the display portion 2705 in FIG. 26) candisplay text and the left display portion (the display portion 2707 inFIG. 26) can display images.

FIG. 26 illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power supply switch 2721, an operation key 2723, aspeaker 2725, and the like. The page can be turned with the operationkey 2723. Note that a keyboard, a pointing device, and the like may beprovided on the same plane as the display portion of the housing.Further, a rear surface or a side surface of the housing may be providedwith an external connection terminal (an earphone terminal, a USBterminal, a terminal which can be connected with a variety of cablessuch as an AC adopter or a USB cable, and the like), a storage mediuminserting portion, or the like. Moreover, the electronic book reader2700 may have a function of an electronic dictionary.

Further, the electronic book reader 2700 may send and receiveinformation wirelessly. Desired book data or the like can be purchasedand downloaded from an electronic book server wirelessly.

Embodiment 11

A semiconductor device of an embodiment of the present invention can beapplied to a variety of electronic appliances (including game machines).As the electronic appliances, for example, there are a television device(also called a television or a television receiver), a monitor for acomputer or the like, a camera such as a digital camera or a digitalvideo camera, a digital photo frame, a cellular phone (also called amobile phone or a mobile telephone device), a portable game console, aportable information terminal, an audio playback device, and a largegame machine such as a pachinko machine.

FIG. 27A illustrates an example of a television device 9600. A displayportion 9603 is incorporated in a housing 9601 of the television device9600. The display portion 9603 can display images. Here, the housing9601 is supported on a stand 9605.

The television device 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller 9610. The channel andvolume can be controlled with operation keys 9609 of the remotecontroller 9610 and the images displayed on the display portion 9603 canbe controlled. Moreover, the remote controller 9610 may have a displayportion 9607 on which the information outgoing from the remotecontroller 9610 is displayed.

Note that the television device 9600 is provided with a receiver, amodem, and the like. With the receiver, general television broadcastingcan be received. Moreover, when the display device is connected to acommunication network with or without wires via the modem, one-way (froma sender to a receiver) or two-way (e.g., between a sender and areceiver or between receivers) information communication can beperformed.

FIG. 27B illustrates an example of a digital photo frame 9700. Forexample, a display portion 9703 is incorporated in a housing 9701 of thedigital photo frame 9700. The display portion 9703 can display a varietyof images, for example, displays image data taken with a digital cameraor the like, so that the digital photo frame can function in a mannersimilar to a general picture frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection terminal (such as a USB terminal or aterminal which can be connected to a variety of cables including a USBcable), a storage medium inserting portion, and the like. They may beincorporated on the same plane as the display portion; however, they arepreferably provided on a side surface or the rear surface of the displayportion because the design is improved. For example, a memory includingimage data taken with a digital camera is inserted into the storagemedium inserting portion of the digital photo frame and the image datais imported. Then, the imported image data can be displayed on thedisplay portion 9703.

The digital photo frame 9700 may send and receive informationwirelessly. Via wireless communication, desired image data can bewirelessly imported into the digital photo frame 9700 and displayed.

FIG. 28A illustrates a portable game console including a housing 9881and a housing 9891 which are jointed with a connector 9893 so as to beopened and closed. A display portion 9882 and a display portion 9883 areincorporated in the housing 9881 and the housing 9891, respectively. Theportable game console illustrated in FIG. 28A additionally includes aspeaker portion 9884, a storage medium inserting portion 9886, an LEDlamp 9890, an input means (operation keys 9885, a connection terminal9887, a sensor 9888 (having a function of measuring force, displacement,position, speed, acceleration, angular speed, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, current, voltage, electric power,radiation, flow rate, humidity, gradient, vibration, smell, or infraredray), and a microphone 9889), and the like. Needless to say, thestructure of the portable game console is not limited to the above, andmay be any structure which is provided with at least a semiconductordevice according to an embodiment of the present invention. Moreover,another accessory may be provided as appropriate. The portable gameconsole illustrated in FIG. 28A has a function of reading a program ordata stored in a storage medium to display it on the display portion,and a function of sharing information with another portable game consolevia wireless communication. The portable game console of FIG. 28A canhave a variety of functions other than those above.

FIG. 28B illustrates an example of a slot machine 9900, which is a largegame machine. A display portion 9903 is incorporated in a housing 9901of the slot machine 9900. The slot machine 9900 additionally includes anoperation means such as a start lever or a stop switch, a coin slot, aspeaker, and the like. Needless to say, the structure of the slotmachine 9900 is not limited to the above and may be any structure whichis provided with at least a semiconductor device of an embodiment of thepresent invention. Moreover, another accessory may be provided asappropriate.

FIG. 29A illustrates an example of a cellular phone 1000. The cellularphone 1000 includes a housing 1001 in which a display portion 1002 isincorporated, and moreover includes an operation button 1003, anexternal connection port 1004, a speaker 1005, a microphone 1006, andthe like.

Information can be input to the cellular phone 1000 illustrated in FIG.29A by touching the display portion 1002 with a finger or the like.Moreover, calling or text messaging can be performed by touching thedisplay portion 1002 with a finger or the like.

There are mainly three screen modes of the display portion 1002. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting information such as text. Thethird mode is a display-and-input mode in which two modes of the displaymode and the input mode are mixed.

For example, in the case of calling or text messaging, the displayportion 1002 is set to a text input mode mainly for inputting text, andtext input operation can be performed on a screen. In this case, it ispreferable to display a keyboard or number buttons on almost the entirescreen of the display portion 1002.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside thecellular phone 1000, display on the screen of the display portion 1002can be automatically switched by judging the direction of the cellularphone 1000 (whether the cellular phone 1000 is placed horizontally orvertically for a landscape mode or a portrait mode).

Further, the screen modes are switched by touching the display portion1002 or operating the operation button 1003 of the housing 1001.Alternatively, the screen modes can be switched depending on kinds ofimages displayed on the display portion 1002. For example, when a signalfor an image displayed on the display portion is data of moving images,the screen mode is switched to the display mode. When the signal is textdata, the screen mode is switched to the input mode.

Further, in the input mode, a signal is detected by an optical sensor inthe display portion 1002 and if input by touching the display portion1002 is not performed for a certain period, the screen mode may becontrolled so as to be switched from the input mode to the display mode.

The display portion 1002 can also function as an image sensor. Forexample, an image of a palm print, a fingerprint, or the like is takenby touching the display portion 1002 with the palm or the finger,whereby personal authentication can be performed. Moreover, when abacklight or sensing light source which emits near-infrared light isprovided in the display portion, an image of finger veins, palm veins,or the like can be taken.

FIG. 29B illustrates another example of a cellular phone. The cellularphone in FIG. 29B has a display device 9410 provided with a housing 9411including a display portion 9412 and operation buttons 9413, and acommunication device 9400 provided with a housing 9401 includingoperation buttons 9402, an external input terminal 9403, a microphone9404, a speaker 9405, and a light-emitting portion 9406 that emits lightwhen a phone call is received. The display device 9410 which has adisplay function can be detachably attached to the communication device9400 which has a phone function in two directions represented by theallows. Thus, the display device 9410 and the communication device 9400can be attached to each other along their short sides or long sides. Inaddition, when only the display function is needed, the display device9410 can be detached from the communication device 9400 and used alone.Images or input information can be transmitted or received by wirelessor wire communication between the communication device 9400 and thedisplay device 9410, each of which has a rechargeable battery.

This application is based on Japanese Patent Application serial no.2008-258992 filed with Japan Patent Office on Oct. 3, 2008, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising a pixel portion and adriver circuit, wherein the driver circuit comprises: a first transistorincluding a first oxide semiconductor layer, a first gate electrode anda first gate insulating layer between the first oxide semiconductorlayer and the first gate electrode; a second transistor including asecond oxide semiconductor layer and a second gate electrode and asecond gate insulating layer between the second oxide semiconductorlayer and the second gate electrode; and a wiring electrically connectedto the first oxide semiconductor layer and the second oxidesemiconductor layer, wherein a length of the first oxide semiconductorlayer in a first channel length direction is larger than a length of thefirst gate electrode in the first channel length direction, wherein alength of the second oxide semiconductor layer in a second channellength direction is smaller than a length of the second gate electrodein the second channel length direction, wherein the second gateinsulating layer has an opening over the second gate electrode, andwherein the wiring is in direct contact with the second gate electrodethrough the opening.
 2. The display device according to claim 1, whereineach of the first oxide semiconductor layer and the second oxidesemiconductor layer includes a region with a depression.
 3. The displaydevice according to claim 1, wherein each of the first oxidesemiconductor layer and the second oxide semiconductor layer compriseindium, gallium, and zinc.
 4. The display device according to claim 1,wherein each of the first transistor and the second transistor is ann-channel thin film transistor.
 5. The display device according to claim1, wherein the pixel portion and the driver circuit are provided over asame substrate.
 6. The display device according to claim 1, wherein thepixel portion includes a liquid crystal layer.
 7. The display deviceaccording to claim 1, wherein the pixel portion includes anelectroluminescent layer.
 8. The display device according to claim 1,wherein the pixel portion includes an electrophoretic layer.
 9. Thedisplay device according to claim 8, further comprising an operationbutton, wherein the display device is an electronic paper.
 10. Anelectronic appliance comprising the display according to claim 1,further comprising an operation button, a speaker, and a microphone. 11.The electronic appliance according to claim 10, wherein the electronicappliance is a cellular phone.
 12. A display device comprising a pixelportion and a driver circuit, wherein the driver circuit comprises: afirst transistor including a first oxide semiconductor layer, a firstgate electrode and a first gate insulating layer between the first oxidesemiconductor layer and the first gate electrode; a second transistorincluding a second oxide semiconductor layer and a second gate electrodeand a second gate insulating layer between the second oxidesemiconductor layer and the second gate electrode; a first wiringelectrically connected to the first oxide semiconductor layer and thesecond oxide semiconductor layer, and a second wiring electricallyconnected to the second oxide semiconductor layer, wherein a length ofthe first oxide semiconductor layer in a first channel length directionis larger than a length of the first gate electrode in the first channellength direction, wherein a length of the second oxide semiconductorlayer in a second channel length direction is smaller than a length ofthe second gate electrode in the second channel length direction,wherein the second gate insulating layer has an opening over the secondgate electrode, and wherein the second wiring is in direct contact withthe second gate electrode through the opening.
 13. The display deviceaccording to claim 12, wherein each of the first oxide semiconductorlayer and the second oxide semiconductor layer includes a region with adepression.
 14. The display device according to claim 12, wherein eachof the first oxide semiconductor layer and the second oxidesemiconductor layer comprise indium, gallium, and zinc.
 15. The displaydevice according to claim 12, wherein each of the first transistor andthe second transistor is an n-channel thin film transistor.
 16. Thedisplay device according to claim 12, wherein the pixel portion and thedriver circuit are provided over a same substrate.
 17. The displaydevice according to claim 12, wherein the pixel portion includes aliquid crystal layer.
 18. The display device according to claim 12,wherein the pixel portion includes an electroluminescent layer.
 19. Thedisplay device according to claim 12, wherein the pixel portion includesan electrophoretic layer.
 20. The display device according to claim 19,further comprising an operation button, wherein the display device is anelectronic paper.
 21. An electronic appliance comprising the displayaccording to claim 12, further comprising an operation button, aspeaker, and a microphone.
 22. The electronic appliance according toclaim 21, wherein the electronic appliance is a cellular phone.